What is a Clerihew poem?

A Clerihew poem is a four-lined humorous poem that follows a specific rhyme scheme and meter. It was invented by English writer Edmund Clerihew Bentley in the early 20th century. It is typically 4 lines in iambic tetrameter with an AABB rhyme scheme. For example:

Sir Christopher Wren

Said, "I am going to dine with some men.

If anybody calls,

Say I am designing St Paul's."