Alice, Betty and Cathy live in the same building, in apartments A, B and C. They plan to take a long vacation together, but they know the power is unreliable in their building, so before they go, they make sure their freezers have no perishable food.
"I wonder if the power will go out while we're gone," says Alice, "I guess there's no way to tell."
"There is!" says Betty, "and I'll know when we get back."
Two weeks later, they arrive home, and each runs to check her freezer.
"I looked in my freezer and it was running, and there was frost on the walls," said Alice. "I guess the power was on the whole time."
"Well I looked in my freezer, and I know there was at least one power outage," says Betty.
"In fact, there were at least two power outages," says Cathy.
How did Betty know there had been a power outage? How did Cathy know there had been two power outages?
I give up, show me the solution.