How does anyone begin to write a scientific proof about anything?

I wanted to write about something simple but I've realized I'm completely stumped and I feel really stupid. I've had writer's-block before but this is worse.

Science is good at one thing, and that's telling people what is actually a fact or not. Right? See pic related, this simple game is something I already know for a fact, the square peg CANNOT go through the round hole. It's for babies yet when I try to put the game into words I come up with nothing.

OK, that's great. But it doesn't feel like proof to me. What am I missing?