"Mr. Impossible could do the most amazing things."
William meets Mr. Impossible on the way to school one day. As they walk together, William asks Mr. Impossible a series of questions, and each answer is follwed by an impossible feat.
Mr. Impossible goes with him to the school, and continues to impress everyone with his impossible tricks. At the end of the book, William tells his parents about his new friend, and they say,
"That's Impossible." ;)
I loved the Mr. Men and Little Miss series in the 70s when I was just a kid, and it positively blew my mind when I found out they has been reprinted.
Another one that children can relate to is Mr. Noisy (Mr. Men and Little Miss).