Nisnevich topology

Okay, so imagine you have a big box filled with lots of little building blocks. You can stack them all up to make towers or other shapes. Now, let's say some of these blocks are painted red and others are painted blue. You want to figure out which of these towers or shapes can be made only using the red blocks, and which can be made using both red and blue blocks.

This is kind of like what people do with math and topology. They use a thing called a space, which is like a big box with lots of different points in it. Imagine you're looking at this space from really far away and you can't see all the little details--kind of like when you look at a city from an airplane. But instead of blocks, this space has things called sets, which are kind of like groups of points in the space.

Now, a Nisnevich topology is a way of figuring out which sets can be made only using certain points in the space and which sets can be made using both those points and other points in the space. It's like separating out the sets into different groups based on what points they use.

Think about it like this: imagine you have a bunch of different toys, like a ball, a doll, and a stuffed animal. You can put them in different baskets based on what they have in common. For example, you might put the ball and the stuffed animal in one basket because they're both soft and squishy, but put the doll in a different basket because it's harder and doesn't squish as easily. The Nisnevich topology works kind of like that--it separates sets into different "baskets" based on what points they have in common.

So, in short, a Nisnevich topology is a way of making sense of different sets of points in a space by separating them into groups based on what points they have in common. It's like sorting toys into different baskets based on what they're made of or how they feel.