Le Caméléon

Everyone knows the rule. Except one.

Everyone gets the same behaviour rule. One person never sees it.

The goal

The Chameleon has to blend in. The group has to unmask them.

The phone goes round privately
Each player reads the secret rule alone. One randomly chosen player never sees it: the Chameleon — and nobody knows who that is.
The conversation starts
The app shows a topic. Everyone speaks in turn, phone down, applying the rule as naturally as they can. Change topic as often as you like.
The vote
When the doubt sets in, you vote. The phone travels: each player picks someone in secret, with no way for the next player to see that choice.
The reveal
You see who voted for whom, then who the Chameleon was.

The group wins by naming the right player. But an unmasked Chameleon gets one last shot: state the exact rule and they steal the round anyway.

Don’t overplay the rule — that is the fastest way to look guilty when you don’t have it.
Chameleon: speak late in the turn and copy the shape of other people’s answers.
Group: whoever stays vague every single turn usually doesn’t know the rule.