This prank will both annoy and confuse your victim, causing them to question their very sanity (if they're paranoid enough). With a bit of luck, or an acquired skill at misdirection, you can pull it off!
After you follow these simple steps, the victim will wind up unsuspectingly drinking Tabasco or another brand of hot sauce from their beverage, but no one else will be able to taste it, leaving your victim both looking and tasting off-kilter.
Warning! Don't try this on anyone with any sort of medical situation which might result in them having any sort of attack; I'm not responsible for your actions.
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Step 1: Situation and Materials
This prank require a specific set-up. Your victim must be drinking something from any kind of container through a removable straw. The only thing that you'll need to supply is the hot sauce, and maybe a distraction, although this can be done if the person is either already distracted (i.e., in a heated discussion or trying to pick up a girl) or if they have excused themselves to go to the restroom. One more suggestion for preparation, please try to wash your hands sometime before enacting this prank; no need to add insult to injury.
Step 2: Step One
While your (ex-) friend isn't watching, remove the straw from their drink and simply dip it into the hot sauce, covering the top of the straw with a finger before removing it, so that a good amount of hot sauce is in the straw.
Step 3: Step Two
Place it back into the drink and, when the straw is all the way down and a good portion of it is under the liquid, release your finger hold. Since most of the beverages that we drink are water-based, both your victim's beverage of choice and the hot sauce will have similar densities, and so the hot sauce will remain in the straw instead of releasing into the rest of the cup and simply mixing with the beverage.
Step 4: The Result
The next time that your victim takes a sip, they will more than likely be shocked and a bit appalled to find that they have just sucked up something unexpectedly spicy. But when others try the drink, they will taste only the ordinary drink. The second time your friend drinks it, it will be back to normal, leaving them confused, perplexed, annoyed, and with a flaming tongue. The first sip after your tampering will be the only bad one, as they will have sucked up all of the hot sauce in that one go.
Congratulations, you're a jerk-weed! Applause all around!
Step 5: How Does It Work?
As previously explained, the liquids don't mix because they are relatively the same density; neither one sinks or floats in relation to the other. However, that's not all that there is to this story. The liquid stays in the straw when you have it covered with your finger because forces are acting on the liquid that are not greater than the pull of gravity at this point. Covering the top of the straw creates a vacuum, as air pressure is no longer a factor inside of the straw; normally, air pressure presses on the liquid from both ends of the straw, and gravity lets the air pushing downwards win.
Additionally, the relatively minor forces of water's natural properties have some small effect. You may notice that the hot sauce will dip slightly under the uncovered edge of the straw, creating a convex sort of half-bubble called the meniscus. Unseen by us, unless the straw is clear, the same volume of liquid is gone from a concave surface of liquid at the other end. This occurs because water is both adhesive (sticking to the sides of the straw) and cohesive (sticking to itself and keeping molecules together).
All of these factors allow you to be a jerk to your friends. Aren't the sciences fun?