Brain Teasers wanted

[AbelM]

For a riddle in the magazine of my student association (physics, mathematics and computer science students) i'm looking for some probability riddles. A good example would be this one which was posted some time ago in this forum.

Another good example would be the famous "three door problem"

Does anyone know some similar riddles? It's quite important that they aren't well known and not easily found on google.

Thanks in advance!