This is a case where a picture is worth a thousand words. The slicing intuition is quite useful and I imagine quite common for those who have taken calculus. It

This is a case where a picture is worth a thousand words. The slicing intuition is quite useful and I imagine quite common for those who have taken calculus. It is the very same mental picture as you would use for computing area using two nested integrals. This video [1] has some nice graphics along these lines. [1] https://www.youtube.com/watch?v=gifQWtTWqEY https://www.youtube.com/watch?v=gifQWtTWqEY