Surface equivalence principle

The surface equivalence principle is like a rule that tells us that things that look the same on the outside can actually be very different on the inside. For example, an apple and an orange might look very different if you cut them open and look at their insides. But if you just look at them from the outside, they look pretty much the same. This principle helps us understand how things can look the same on the outside but be very different on the inside.