"What do you mean, he rides his bike like lightning?"
"He's always hitting trees!"
Today's joke is based on the expression ...like lightning. We usually use this metaphor to mean that something happens quickly or fast. For example:
'I ran like lightning to the bus-stop.' = I ran very quickly to the bus-stop.
'She drives like lightning'- She drives her car fast.
In this joke the meaning of like lightning is changed to mean that the boy on the bike hits trees as often as lightening does in a storm!