A little pressurised gas cannister in a can of beer, especially exported English and Irish beers. Upon opening the can, the change in pressure causes the gas (usually CO2) to be released from the cannister hence making the beer bubbly, or carbonating it if you prefer. Solves the problem of the beer going flat when exported.

Or something like that. I don't know much about beer and I wouldn't want to spoil all your guessing fun.