What is unusual about Tock in The Phantom Tollbooth?

The most unusual thing about Tock the bio-mechanical watchdog is that he can only make a ticking sound.
This is not how things were supposed to turn out. When Tock's parents gave birth to him, they already had a clock puppy called Tick. They gave him this name in the hope that he'd make a ticking sound. But when they wound him up they found that he could only go "tock." So they figured out that if they had another child, they would name him Tock, in the hope that this time they'd have a pup that would make the correct sound.
It didn't work like that, however, and so Tick and Tock's parents have ended up with two pups, neither of whom can make the right sound: Tick can only tock, whereas Tock can only tick.