Another cool optical illusion is the classic ABC 12 13 14 eye trick. Your mind will perceive the B as a letter when you are reading across. But when you read the list of numbers, it looks like twelve, thirteen, fourteen.
You can try it with different fonts, as well as just writing it with a pencil or Sharpie.
To trick your friends: Draw (or print) the A B C 12, 13, 14 picture. Then use another piece of paper to cover up everything but the letter B/13. Ask your friend what it is. If they say 13, then show them the ABC. If they say B, show them the 12, 13, 14. You win either way, because it’s cool!