Which number is missing?

A nursery school teacher was delivering a station wagon full of kids home one day when a fire truck zoomed past. Sitting in the front seat of the fire truck was a Dalmatian dog. The children fell to discussing the dog's duties.
"They use him to keep crowds back," said one youngster.
"No," said another, "he's just for good luck."
A third child brought the argument to a close. "They use the dogs," she said firmly, "to find the fire hydrant."