### Star bright

So every year, Zenella is saying, we add another year on to the time since the dinosaurs disappeared. If it was 65 million years last year, it's 65 million and one this year.

Yes, I say, I suppose there was a year when there were no more dinosaurs, or the last of them evolved into something else.

Although you could argue that they didn't, because birds are a sort of dinosaur, they say. But maybe a dinosaur only in the same way I am a marmoset.

Sixty-five million years! How to explain to a child who has no concept that her life has a span that that is a million lives? It's impossible to conceptualise. You cannot picture the lives one after the other. Even picturing the faces of your ancestors (imagining them, I mean) is an enormous task. (And what do you picture? Are you seeing lords when you reach three figures or are you seeing rough men and women in clothes little more structured than coal sacks? I find it interesting that when people trace back their family, they always find some distant cousin who was somebody. Go back far enough and we're all cousins: all Europeans descended from something like seven men, and all of them probably descended from the same monkey.)

If you step outside a spaceship, Zenella is saying, what happens?

You have to be tied on with a lifeline, I say, and I explain what that is.

If you fell off, she is saying, you have to hope you are near a planet and land on it.

But you would be set on fire by entering its atmosphere, I say.

Why? she is saying, and I explain that too, although I'm not sure whether if a planet has a very thin atmosphere it would be enough to set you on fire. You might just splish on the surface.

Her tennis coach says, Yeah, if there's gravity.

All planets have gravity, I say.

He is confused because the moon has less gravity than the Earth, and those high-bounding spacemen have made him think that it has none at all.

I tried to explain to Mrs Zen the other night that she too had gravity. I stumbled when trying to explain what mass is. It's difficult to explain without using the word "weight". Eventually, I said that an object's mass is the amount of smash when it hits something else, which is true enough.

I wanted to talk to her about Bayes' theorem. I've been studying it, and have a faint understanding of it. But she didn't want to know.

(On the offchance that someone reading this wants to know, Bayes' theorem helps you determine the importance of new pieces of evidence. It's important in thinking about poker. Say someone raises preflop, and you want to know whether he has a high pair. Bayes' theorem tells us:

The probability that he has a high pair given that he raised is equal to the probability that he will raise given a high pair times the probability that he has a high pair, divided by the same plus the probability that he will raise without a high pair times the probability that he doesn't have a high pair

or p(A|B) = p(B|A)p(A)/p(B|A)p(A)+p(B|~A)p(~A)

where A is having a high pair, B is raising.

It's one of those things that looks hard to understand until you understand it!

Let's say he is dealt a high pair 10% of the time and will raise with it 80% of the time and will raise with other stuff 10% of the time.

The chances a raise means a high pair are: .8x.1/.8x.1 + .1x.9 = .08/.17, or 47%. If he raises 20% of his other stuff, the chances that he has a high pair become a bit less than 30%. How much a piece of evidence can lead you to draw a particular conclusion depends on how much that piece of evidence could mean a different conclusion.)

Mrs Zen could do with learning Bayes' theorem because she tends simply to add pieces of evidence together and draw conclusions, instead of weighting her evidence by considering what other things it could be evidence of. She has a confirmation bias. Whenever she has evidence of evildoing on my part, she assumes I've done evil. However, some of the time, that evidence of evildoing is an outcome of perfectly innocent behaviour.

But Mrs Zen assumes that the probability of my doing evil is 100% anyway.

It is no wonder creationists cannot stand geological time. How is it possible that their god can think them special if three million generations have passed since the end of the dinosaurs? How can he have wanted them personally to exist?

Zenella is keeping a chart of the phases of the moon. The crescent is in the wrong place here. My dad noticed it. It's at the bottom of the moon, rather than at one side.

Zenella asks me what stars are made of. I explain but I think she is struggling to grasp how a colourless gas can be so bright. She does not understand that our sun is small.

But it's hard to believe it's small when it's so much bigger than anything else in our lives.

Yes, I say, I suppose there was a year when there were no more dinosaurs, or the last of them evolved into something else.

Although you could argue that they didn't, because birds are a sort of dinosaur, they say. But maybe a dinosaur only in the same way I am a marmoset.

Sixty-five million years! How to explain to a child who has no concept that her life has a span that that is a million lives? It's impossible to conceptualise. You cannot picture the lives one after the other. Even picturing the faces of your ancestors (imagining them, I mean) is an enormous task. (And what do you picture? Are you seeing lords when you reach three figures or are you seeing rough men and women in clothes little more structured than coal sacks? I find it interesting that when people trace back their family, they always find some distant cousin who was somebody. Go back far enough and we're all cousins: all Europeans descended from something like seven men, and all of them probably descended from the same monkey.)

If you step outside a spaceship, Zenella is saying, what happens?

You have to be tied on with a lifeline, I say, and I explain what that is.

If you fell off, she is saying, you have to hope you are near a planet and land on it.

But you would be set on fire by entering its atmosphere, I say.

Why? she is saying, and I explain that too, although I'm not sure whether if a planet has a very thin atmosphere it would be enough to set you on fire. You might just splish on the surface.

Her tennis coach says, Yeah, if there's gravity.

All planets have gravity, I say.

He is confused because the moon has less gravity than the Earth, and those high-bounding spacemen have made him think that it has none at all.

I tried to explain to Mrs Zen the other night that she too had gravity. I stumbled when trying to explain what mass is. It's difficult to explain without using the word "weight". Eventually, I said that an object's mass is the amount of smash when it hits something else, which is true enough.

I wanted to talk to her about Bayes' theorem. I've been studying it, and have a faint understanding of it. But she didn't want to know.

(On the offchance that someone reading this wants to know, Bayes' theorem helps you determine the importance of new pieces of evidence. It's important in thinking about poker. Say someone raises preflop, and you want to know whether he has a high pair. Bayes' theorem tells us:

The probability that he has a high pair given that he raised is equal to the probability that he will raise given a high pair times the probability that he has a high pair, divided by the same plus the probability that he will raise without a high pair times the probability that he doesn't have a high pair

or p(A|B) = p(B|A)p(A)/p(B|A)p(A)+p(B|~A)p(~A)

where A is having a high pair, B is raising.

It's one of those things that looks hard to understand until you understand it!

Let's say he is dealt a high pair 10% of the time and will raise with it 80% of the time and will raise with other stuff 10% of the time.

The chances a raise means a high pair are: .8x.1/.8x.1 + .1x.9 = .08/.17, or 47%. If he raises 20% of his other stuff, the chances that he has a high pair become a bit less than 30%. How much a piece of evidence can lead you to draw a particular conclusion depends on how much that piece of evidence could mean a different conclusion.)

Mrs Zen could do with learning Bayes' theorem because she tends simply to add pieces of evidence together and draw conclusions, instead of weighting her evidence by considering what other things it could be evidence of. She has a confirmation bias. Whenever she has evidence of evildoing on my part, she assumes I've done evil. However, some of the time, that evidence of evildoing is an outcome of perfectly innocent behaviour.

But Mrs Zen assumes that the probability of my doing evil is 100% anyway.

It is no wonder creationists cannot stand geological time. How is it possible that their god can think them special if three million generations have passed since the end of the dinosaurs? How can he have wanted them personally to exist?

Zenella is keeping a chart of the phases of the moon. The crescent is in the wrong place here. My dad noticed it. It's at the bottom of the moon, rather than at one side.

Zenella asks me what stars are made of. I explain but I think she is struggling to grasp how a colourless gas can be so bright. She does not understand that our sun is small.

But it's hard to believe it's small when it's so much bigger than anything else in our lives.