Horses

S was getting something for K, and he was impatient. "Hold your horses," I told him.
He answered, "Forty-nine."
Puzzled, I asked him, "What's forty-nine?"
"It's a number," he explained patiently.
"OK, I know it's a number," I said, "but why did you SAY 'forty-nine'?"
Kivi answered, "Because that's how old my horses are!"