The gas station

It was the winter holidays, and Jack’s family decided to travel to Alaska. They were on a road trip in a rented car, when Jack’s father, Mr. Watson, noticed the blinking lights on his car. “Time for a refill,” he muttered. It was a foreign place, which was different from where they lived. For a start, it was cold. Very cold. Jack’s mother swore, “It’s minus fifty degrees out there, so help me Frank, I will not get out of that car.” Frank was Jack’s father’s name. Jack plugged in the oil and his father paid the money.

They got in the car.

It wouldn’t move.

“Oh come on, ” Mr. Watson said. “Did you plug in the wrong one? We run on oil, not diesel.”

“No, I’m certain I didn’t.”

They went up to the store in the gas station. After hearing the story, the cashier said: “I’m sorry, the oil must’ve frozen. We don’t accept refunds, but I can give you a discount on our premium oil which doesn’t freeze above minus two hundred degrees!”

“No you won’t,” chirped in Jack. “The liquid you sold us isn’t oil!”

How did Jack know?