Step 1: Build a Tower
Build a tower. It doesn't matter what blocks you use, or the height, but make it pretty tall.
Step 2: Floating Block
Break all the blocks except for the top one.
Step 3: Spawn Chickens
Spawn a ton of chickens on the floating block. They will all stay clumped up in a ball on the top. Be sure that you click on the corner of the block to spawn them, and don't click the chickens or else they will explode.
Step 4: Break the Block
Break the block but be careful not to touch the chickens.
Step 5: Boom!
They will float down in a clump and when they hit the ground they'll explode.
Step 6: Other Stuff
You can also hit them or shoot them with a bow while they are on the block or floating down. Sadly this only works with chickens.
Step 7: Pest Extermination
If you don't want a bunch of chickens lagging up your world, gather them up with seeds and then place some lava. They will all die.