Heads or tails on the board, where the whole class can see it land. No sign-up, nothing to install.
Press the button. The coin turns for a moment and lands on heads or tails, large enough to settle the question from the back of the room. A running count of both sides sits under it, which is the honest answer to "it always gives tails".
It is a straight 50/50 each time, drawn fresh from the browser's random source. Previous flips do not influence the next one, and that is worth saying out loud in a maths lesson.
Learnify puts classroom points and growing characters on the class screen: a point in one tap, seen by everyone, plus attendance, groups, boss battles and quiz games. Pupils need no devices and no accounts of their own.