Forces

Brody just wandered into my room, 20 minutes after we put him to bed. He looked confused and sleepy, squinting at the bright lamp beside the bed. He looked up at me and quietly said "I can only remember 4 of the 5 forces. What's the 5th?" I must have looked confused because he continued, "There's gravity that keeps you on the ground, friction that makes things warm, push, and pull. What's the other one?" I have absolutely no idea and I tell him that. We both look at my phone and without another word, I start googling.

He's 7 years old and already there are questions I can't answer.