As an introduction to chemical changes,  we produce O2 and H2 gas in a plastic bag.  We activate the mixture with a match, and BOOM, a tiny amount of H2O is produced.
Here, Ray has the match and Heidi is the one with the H2 & O2.  Even though the bag isn't very big, the explosion was one to remember!

Reactions that produced the gases:
Zn+HCl==>H2+ZnCl2
H2O2+MnO2==>H2O+O2+MnO2

The explosion reaction:
H2+O2==>H2O                    MORE ABOUT THIS REACTION HERE