Minecraft wiki entry for Slime_Block, associated text: A player or mob that falls onto the top of a slime block bounces to a height proportional to the falling velocity. The bounce height quickly deteriorates. For example, a fall of 255 blocks produces a bounce height of about 50 blocks, while a fall of 50 blocks results in a bounce height of 22 blocks. The maximum bounce height is 57.625 blocks. Entities hitting the side of a slime block do not bounce, unless the slime block is moving using a piston.