While a piston head is moving a slime block horizontally or downward, any mob, boat or minecart that is on the slime block falls through it. Boats and minecarts get stuck inside if another solid block is under the slime block; mobs (at least skeletons, adult zombies and creepers) suffocate if they fall into a 1-block hole under the slime. Likewise, slime blocks rise through a boat or minecart when a piston is retracting upward, but not when it's extending upward. I believe this