The secret of the aluminum can: what is it hiding

What secret is an ordinary aluminum soda can hiding? Reagents and equipment: soft drink in an aluminum can, sandpaper, beaker, drain cleaner, wooden rod, glass container. Buff the top layer of paint off of an aluminum can. Open the can and use a wooden rod to suspend it in a suitably-sized beaker. Add some drain cleaner (usually a 10% sodium or potassium hydroxide solution). Observe a tumultuous reaction and release of gas. Wait two hours. Take the can out of the liquid. It would seem that the aluminum has