• Draw a line between two dots. A new dot appears on it.

• Lines can't cross each other or themselves.

• No dot can hold more than three lines (a loop counts as two).

Send a friend the link and you both play live. Whoever can't move loses.

Invented in Cambridge in 1967 by John Conway and Michael Paterson. A game runs at most 8 moves, but it's deep enough to argue about.