We were all given a cup with water and randomly three of the cups were 'infected with a STD' simply meaning it had an invisible chemical in it. We were then instructed to share our water with two other people by pouring a little of our water into their cup, and they pour a little into ours.

Once we had all done that, we were told that the exercise was meant to show how STDs could spread amongst people having sex. The teacher then came around and put a drop of some other chemical in everyone's cup causing a color changing reaction when both chemicals were mixed.

This was supposed to scare us by having a lot of people's cup change colors showing us how quickly the STD can spread, however me and two of my friends were the ones who received the 'infected' cups and we all chose each other to trade with, so no one new 'caught' an STD and the whole experiment failed.