Soda

Don't get the joke? Well, here's a simple explanation! Tonic water contains quinine, commonly known as a compound that treats malaria. Everyone thinks that quinine tastes bitter, but the bitterness is detected only by the people with at least one copy of a specific bitter taste receptor allele. Others, who have two copies of a different allele, don't taste quinine because quinine doesn't activate the receptors.*

In easy terms: Let's say you have a velcro shirt. Well, cotton balls are going to stick to you. Your roommate, however, has a silk shirt, so cotton balls just fall off.  If you have only half a velcro shirt, the number of cotton scraps on your shirt will vary, but they may still there. Therefore, if you've been tampering with you roommate's cotton balls, we'll see the evidence on your shirt!

In my comic, the roommate mixed some tonic water with his soda, since he doesn't taste the bitterness of quinine. His roommate, however, DOES have the allele that detects bitterness!

You can try this experiment among your friends (the larger the group, the better)! Buy 1L of tonic water (It's less than a dollar in the US). Nowadays, supermarket tonic water has a lots of sugar/HFCS, some citric acid, and only a minute amount of quinine, but people can still detect the quinine. Ask everyone to take a sip of the tonic water. Those who detect quinine will pucker up, exclaim "This is dis-GUS-ting!", while others will mumble "I don't get it... it tastes like [lemon-lime soda] to me..."

* The perception of quinine taste intensity is associated with common genetic variants in a bitterreceptor cluster on chromosome 12.

Reed DR, Zhu G, Breslin PA, Duke FF, Henders AK, Campbell MJ, Montgomery GW, Medland SE, Martin NG, Wright MJ.

I'm research bitter taste receptors under one of these scientists, actually!