The story given to the girls is that a group of hostile aliens is trying to steal important things from Earth. Since nothing is more important to Guides than our promise, we have devised a way to trick the aliens. We are going to translate the promise into an alien proof code.
The way that the code works is that you take each part of the promise and translate it into its opposite. For example, "I will help others" would become "I will be mean and nasty to others and never stop to lead a hand". Girls are given a paper and they translate the entire promise into this alien proof code. When completed, they read out their new promise to the group.
The purpose of this activity is to make the girls think about what each part of the promise means and what behaviour isnt in the promise. It also allows them to be very silly and have a giggle about the opposite promise they have created.