She was generating hydrogen gas. I used to do this all the time when I was in high school and college. The cleaner (lye) reacts with the water to produce a great deal of heat, which strips the oxygen from the water and binds it to the aluminum foil. The hydrogen gas bubbles of. We used to do the reaction in glass pop bottles and put a balloon over the lip. After the balloon filled up sufficiently we'd tie it off, tie some toilet paper on the end, light the paper, and send the balloon out the window. For a bigger bang, we'd blow a little bit of air in first then fill the balloon with the hydrogen. Good times.