system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the last bit of ice in a cup refuse to come out without me having to bang the cup?
gpt
The same thing that gives water its surface tension gives it the ability to stick to other surfaces. Each water molecule is basically a tiny, weak permanent magnet. This allows it to stick to other water molecules pretty well, but it also allows it to stick to other substances since most other substances have a tiny bit of charge, as well. However, this magnetic/sticky force is very weak (this is why some bugs can walk on the surface of water, but you can easily push your finger through the surface. 

So knowing all this, the reason you have to bang on the cup is because the ice is always melting (assuming you're in a room temperature environment), so the ice has a layer of water all around it and on the bottom of the cup. That surrounding water's sticky force is only strong enough to hold the smallest pieces of ice. A second piece of ice sitting on top of the bottom layer of ice doesn't have the advantage of being surrounded so thoroughly by water, so it easily falls out.

tl;dr: only a thin layer of water can stick to the bottom of the cup. If a piece of ice is small/light enough, the bottom layer of water sticks to both the cup and that piece of ice, holding it in place.