This is a working K'nex grenade I threw together with instructions, so you can explode it too!
Step 1: Pieces You Need
Step 2: The Base
Build this and add a white connecter over the top and another arm in mirror on the other side like in the second photo. Then, add a white rod to the bottom and an orange connector to the rod.
Step 3: The Release Mechanism
Put the yellow rod into the orange connecter and then thread on the yellow hemisphere and put on the black stopper.
Step 4: Loading
The shrapnel (six spacers) on the arms and put the hemisphere over them. Finally, put the orange connecter on the top of the yellow rod to act as a safety pin and stretch two loom bands (or just a normal band doubled up) in the holes in each arm.