Sigma additivity

Sigma additivity is a big and fancy word that means we can add up lots of things and get a big total. Imagine you have a bowl of candy and you want to count how many pieces there are. You start with 5 pieces, and then your friend adds 3 more, and then your mom puts in 2 more. If we use sigma additivity, we can add up all these pieces of candy to get the total. So, 5 + 3 + 2 = 10 pieces of candy in the bowl. This works for more than just candy - we can use sigma additivity to add up lots of things!