I'll give you a clue; it's NOT a Ping Pong ball (it's smaller), or anything to do with sport.
I'll give you another clue (below).
Answer: It's the 'widget' from inside a can of Draught Guinness.
During the canning process, liquid Nitrogen is added to the beer, which, when the can is closed, vaporises and expands, and fills this small widget through a tiny hole. When the can is opened the pressure drops and this gas escapes, forming the creamy head of a classic draught Guinness. Pure genius.
So, there you are. If you already knew all this, then count yourself a brain-box. If you didn't, you can thank me for littering your head with yet another bit of totally useless information.