The physics teacher dropped an apple to demonstrate gravity.

The humor in a physics teacher's gravity demonstration going awry relies on a delightful subversion of our expectations. When an object is dropped, especially in a controlled classroom setting, we anticipate it will follow the laws of physics perfectly and land safely on the floor. The punchline, however, introduces an absurd twist where the teacher somehow "misses" the entire floor, redirecting the apple to an unfortunate student's head. This unexpected outcome creates a moment of pleasant surprise and silliness, which is a core mechanism of many jokes.

This joke also playfully nods to one of science's most famous (and possibly apocryphal) origin stories: Isaac Newton and the apple. Legend has it that a falling apple inspired Newton to ponder the force that pulls objects towards the Earth, leading to his groundbreaking theory of universal gravitation. While the apple likely didn't bonk him directly on the head, the image of it falling was enough to spark a revolution in understanding the cosmos. The joke takes this iconic image and gives it a very human, clumsy, and comical spin, reminding us that even in the pursuit of scientific understanding, unexpected bumps (or apples) can occur.