20 questions
One of you gets a secret. The other finds it with up to 20 yes/no questions.
How to play
Choose who knows the secret. If it's your student, send them the link so the secret shows only on their screen. If it's you, press Open the secret in a new tab and read it there, in a tab you're not sharing. The other player asks yes/no questions; press Yes or No after each answer to count down from 20. Then swap roles.