We ran out of milk this evening, so I made an emergency trip to the grocery store to buy more. Generally we purchase a half gallon of one-percent, which lasts us about a week.

When I went to grab the milk from the refrigerator case, however, I was startled by the price: $3.19! Usually we pay between $1.99 and $2.29.

Our of curiosity, I priced the full gallons. They were on sale for $2.99. That’s right: The sale price on a gallon of milk made the unit price less than 50% of the unit price for a half gallon. (That is, an ounce of milk from the gallon would cost me 2.34 cents, whereas an ounce from the half gallon would cost me 4.98 cents.)

I bought the gallon, naturally. We probably won’t drink it all, but by golly, I just saved twenty cents!