Most people hear lye and they immediatly think danger - dont go near it.
My friends and I started playing with lye when we were still in elementary school. I respect it, but I'm not at all afraid of it.
"What in the blazes were we doing playing with lye!?!?!" you might ask.
Get a gallon glass jug, put about 3 inches of water in it, add a bunch of lye (we NEVER measured it). Put a brick or two down into the bottom of a five-gallon bucket, set the set the gallon jug on top of the bricks, get the garden hose and fill the bucket with water. Then turn the water down really low and let it keep flowing into and over the top of the bucket (cold water bath), shove aluminum foil into the jug and put a balloon over the top of the jug.
Aluminum is a VERY strong reducer -- so much so that it can pull the oxygen out of water -- WAY better than iron. But what normally happens in that a layer of aluminum oxide form on the surface of the aluminum and it prevents the rest of it from oxidizing. However, the lye prevents that layer from forming. Rather, the aluminum oxide falls to the bottom of the jug. That process liberates the hydrogen -- which is captured by the balloon.
When the balloon is full, tie it off and let it go -- floats away.
You need the water bath to keep it cool or your balloon will fill with steam instead of hydrogen -- and it will not float away.
Or, let the aluminum fizzle a while and then strike matches and throw them at the top of the jug -- poof.
Either way, lots of fun for young boys.