To begin with get the group to stand in a large circle. Ask each person to choose one person to be their bomb and a second person to be their shield. The two choices must be different people. The choices they make are to be kept a secret.
When the teacher shouts go, the aim of the challenge is for each participant to try and keep their shield between themselves and their bomb. This will almost certainly lead to everyone running around trying to maintain his or her position.
At different points the teacher / facilitator can blow a whistle to pause the game. At this point the imaginary bomb will explode and they can ask different people who their bomb and shield are.
Every time the game is stopped participants will need to re choose their bomb and shield.