Wallis sieve

So imagine you have a giant bag of marbles. Some of these marbles are red and some are blue. You want to figure out how many marbles there are in total, but you don't want to count them one by one because that would take forever!

So instead, you use something called the Wallis sieve. It's like a special tool that helps you count the marbles.

First, you line up all the marbles in rows. Then you draw a line through the middle of each row. You count how many red marbles are on one side of the line and how many blue marbles are on the other side.

Then you do some math. You multiply the number of red marbles by the number of blue marbles on the other side of the line. Then you add up all those numbers from each row.

This gives you an estimate of how many marbles are in the bag. It's not exact, but it's pretty close and much faster than counting them all one by one!