A mother sent her child out to buy some edable silver ballbearings to decorate a birthday cake. Only the child didn't understand and bought steel ballbearings instead.
They all ate the cake but the next morning the first daughter comes downstairs and says "Mum, when I shit this morning I had ballbearings come out of my arse."
The second daughter comes downstairs and says, "Mum, when I cleaned my Tampax out this morning I had ballbearings on it."
The little son comes downstairs and says, "Well, you think that's bad. When I wanked myself this morning I shot the cat!"