This is a highly mature device, popular with schoolchildren

The 'Bomb Bag' is a 4 inch square foil bag, sealed so as to be airtight. Inside is a large quantity of baking soda, and a sachet of acid and water.

To use a bomb bag:
  1. Probe the bag until you locate the sachet
  2. Squeeze the sachet so as to burst it
  3. Secrete the 'Bomb Bag' upon someone else's person (Their bag is good, but a pocket is even better1)
  4. Retire to a safe distance
Once the sachet is popped, the acid begins to react with the baking soda, forming carbon dioxide and water (and a salt).
For example, a 'Bomb Bag' that uses citric acid:
3NaHCO3 (Baking soda) + C6O7H6 (citric acid) -> Na3C6O7H (sodium citrate (the salt)) + 3CO2 (Carbon dioxide) + 3H2O (water)
The carbon dioxide formed causes the pressure inside the bag to rise, until the bag bursts, creating a loud noise, and spraying foamy water everywhere. To the annoyance of the person it's planted on. Bomb Bags can be had for about 10p in any good joke shop.

1 - When I was at school, our school uniform included a blazer which had pockets exactly the right size to conceal 'Bomb Bags', either a supply of undeployed bags, or a single deployed one.