Out of Regular (Gas, That Is)

On the way home from work today, I noticed that I was running low on gas, so I stopped at a Chevron. Unfortunately, each of the gas pumps had a handwritten sign: "Out of regular gas."

"Hmm..." I thought to myself. "I've never seen that before." I decided to go to another Chevron closer to home. When I got there, it's the same: "Out of regular gas."

Penny-pincher that I am, I decided to try a Shell station, just in case it was a Chevron thing. But no--it's the same story--"Out of regular 87 gas." So I finally paid my extra 12 cents per gallon and got "Plus" gas.

So what's going on? Is there some conspiracy to make us buy more expensive gas?