The next time you have some friends over for a cookout, make sure to have plenty of bottled water available for your guests. When a friend drinking one of the bottled waters leaves for the restroom, take some white vinegar and dump about 2 ounces of the sour liquid into their drink before they return. They will get a nasty surprise the next time they go to quench their thirst. This beverage prank also works with soda and juice drinks.
Through personal experience (a coworker really got me with this one!) emptying a glass of regular tap water and filling it with seltzer water gives a real shock to the owner of the glass. I spit a mouthful of bubbly water out all over my desk one morning after returning from the bathroom. My coworker had pulled the old switcheroo by emptying my regular water and replacing it with, as he described, “dirty stale-ass club soda.”
Part of the shock and kicker to this prank is not knowing exactly what you are drinking as the beverage slides down your throat. If you are having trouble coming up with substances to replace or pour into your friend’s drink, here are a few suggestions: salt, corn syrup, acid, vegetable oil, or perhaps some nice Syrup of Ipecac.