improbable = something that can happen, but its probability is comparatively low, but not zero. The distinction between probable and improbable is not, as far as I know, exactly defined.
if you roll one hundred dice, it is very improbable that all of them will land as sixes.
almost impossible = something that can happen, but its probability is exactly zero
if you roll a frictionless die, it is almost impossible that it will end up on its edge.
if you keep rolling a die until a six lands, it is almost impossible that you will never stop rolling.
impossible = something that cannot happen.
if you roll one hundred (standard) dice, it is impossible that at least one of them will land as a seven.
if you do get one hundred sixes in a row, it is probable that the casino will kick you out.
if you attempt to balance a frictionless die on its edge, it will almost definitely topple to one of its faces
I think you will definitely try to balance a die on its edge. If you do succeed - remember it's due to friction.