The Riddle

A prince and his servant are exposed to great danger. They spend a night in a witch's house. The witch however tries to poison them. Luckily they get warned and they slip out. The poison kills only their horse. But the journey does not end there. They stumble upon some robbers who astonishingly die of the poison as well. Then they arrive to a town where a princess wants people to pose riddles for her.
If she solves it, they get killed. If she can't, she marries the one who posed the riddle. And we ask you - do you have any guesses about the riddle our prince and his servant posed and do you think the princess will manage to solve it?