Letter bomb (or also Minesweeper) is a group dynamics game for smaller groups.
20 to 50 letter bombs are required. A bomb is made of an envelope with a vision panel and inserted coloured cards (two different colours) or playing cards with differing colours (e.g. red and black).
The "bombs" (cards) are placed on the ground in such a manner so that the colours are not visible. In order to aid memorising, the cards should be set in a pattern of 5x5- or 6x6.
The group is then given the task. They are allowed to have a look at every single card for a certain time, e.g. 3-5 minutes (one card at the time, meaning, they turn around one card, have a look at the colour, try to memorise and then turn the card to its initial position). Usually, the group may not communicate during that time.
Afterwards, the group leaves the room. They may discuss which team member is going to turn cards. The goal is to only turn around cards that are not bombs (e.g. black ones). By no means, a bomb (the cards in red colour) should be turned around.