Cement Mixer.
1 shot Bailey's Irish Cream
1 shot Lime Juice (or lemon, either is fine)
1 Participant who has never heard of this
You have the victi-, er, participant put a shot of Bailey's Irish Cream in their mouth and start swishing it through their teeth.
Then, without swallowing, they put a shot of lime juice in their mouth with the instructions to keep swishing for the count of 30.
Hilarity ensues as the lime juice curdles the Bailey's Irish Cream. Their swishing evenly distributes the lime, converting the whole mass into a mouthfull of Bailey's Cottage Cheese almost instantly. If they make it through the count of 30, they win. At the end, the person usually spits the mess into a cup, disgusted.
For some reason, it only works once....