After that teacher gives students a letter and several teams or several people have to invent a word for each topic starting with the letter as soon as possible. For example: potato, platypus, piece of metal, pop-music. When you have all the four words you say "stop the bus!" and present your words. The teacher decides whether your words are suitable or not and gives you a point... or not. :) Obviously you have to collect as many points as possible.