Heres a Diet coke and mentos "bomb" prank. Not really a bomb.
Ok all you need is:
any size
bottle of diet coke.
A nail.
dental floss.
Step 1. Poke a nail in the middle of the mentos.
Step 2. Put a string of dental floss through it. Not too short now. 3-5 inches i would suggest.
Step 3. Open the Diet Coke bottle and hang the mentos attached to the dental floss just below the rim of the bottle. So the mentos is underneath the cap.
Step 4. Close the bottle and cut off the remainding dental floss so no one can see that the dental floss is there.
Step 5. Wait till someone opens the bottle and make sure your around to watch this hilarious prank.
Step 6. Watch them open the bottle get scared and laugh your ass off.

Voila