"The Chemistry set" joke

Here's one I heard on the radio recently.
A father comes home and asks where his son is. His wife replies that he's
downstairs playing with his new chemistry set. The father is curious so he
wanders down stairs to see what his son is doing. As he's walking down the
steps he hears a banging sound. When he gets to the bottom he sees his son
pounding a nail into the wall. He says to his son, "What are you doing? I
thought you were playing with your chemistry set. Why are you hammering a
nail into the wall?" His son replied, "This isn't a nail, dad, it's a worm. I
put these chemicals on it and it became hard as a rock."
His dad thought about it for a minute and said, "I'll tell you what
son, give me those chemicals and I'll give you a new Volkswagon." His son
quite naturally said, ''Sure why not.''
The next day his son went into the garage
to see his new car. Parked in the garage was a brand new Mercedes. Just
then his dad walked in. He asked his father where his Volkswagon was. His
dad replied, "It's right there behind the Mercedes. By the way, the Mercedes
is from your mother."
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