In this Instructable, I will show you how to make a grenade from 2 halves of a knex ball(sold in the 250 piece set with 150 bonus parts, and maybe some others, but this is where I got it ), and a spring. I got mine in a nerf gun. Also, a red rod will be helpful.
Let's get started!
Step 1: Assemblance
The red rod is just here to help, you don't really need it, though it is fairly difficult without.
Also, shrapnel may be added, just put it vertically so All the shrapnel won't fall out of one of the halves, then press the two halves together.