A normal set of calipers has 3 basic modes of measuring things: inside, outside, and depth. It is amazing to me how many people in this thread don't know at least one of those or use them wrong.
Tbf people my age and younger (barely below 1/2 the population) typically don't use them in most careers or even learn about calipers in High School. My work was the first place that properly taught me how to measure internal diameter and depth.