EVOLUTION
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Let's say you want to find life on other worlds. This is hard because we don't 
know what we are looking for. So we want to find a way to find life even when 
we don't know what it might look like.

So we want to think of things that life always does. We want to find something 
we can look for that will be there any time there is life, even if that life 
doesn't look like us. The problem is that we only have one kind of life to 
look at, and that's us!

Life changes over time. Some living things die and some living things live, 
because some are better at living. The better-at-living things grow and have 
children and the worse-at-living things die and don't have children. And so 
over time there are more of the better-at-living things. This always happens 
to all life, and it turns out we can use this fact to help find life even 
when we don't know what it looks like!

Let's say you have a hundred different colors of blocks, and you want to use 
them to build living things. It is hard to make each kind of block, so it 
takes less work if you only use some of them, instead of all hundred.

Because of this, living things that make use of only a few colors of blocks 
are better-at-living than things that try to use all the colors of blocks 
at once.

When there is no life, the colors of blocks get picked in a different way.