Fubini's theorem

Fubini's theorem is like a rule for doing math. It helps you work out very complicated equations with multiple variables by breaking them down into smaller parts. Say you have an equation with two variables. With Fubini's theorem, you would break this equation into two smaller equations with just one variable, and then connect them back together so the answer you get includes the two variables. This makes it easier to work out the equation, and gives us the right answer!