The watter mixed with the MRE Heating Pack makes gasses, the gasses expand the bottle to the point of breakage, so when the bottle "Decompresses" it sounds like an explosion simple since here... But loads of fun!
Please excuse the Number of steps, its easier then you think.
Warning - Use at own risk! The gasses from the MRE Heating Pack is very flammable, so I strongly suggest keeping it away from a open flame..
Also, Check with you town law to see if it is legal!
Step 1: Things You Need
You will need --
1-2 MRE Heating packs (use more or less for size of bottle i suggest one or two for a 18-24oz. bottle)
Plastic Bottle with screw on Lid
An some water.. (OH NO NOT WATTER!)