You reach for a pint of chocolate ice cream in the grocery-store freezer - how do you know if it has thawed and refrozen between the manufacturer and your supermarket? A tiny sensor on the package may tell you, if scientists at Sandia National Laboratories have their way.

Patented in April, the postage-stamp-size sensor relies on "smart" materials to indicate whether it has been exposed to temperatures that are too high for the particular product."If a refrigerated truck carrying your frozen TV dinner breaks down and the dinner thaws, then this sensor tells you," said Mo Shahinpoor, a professor of mechanical engineering at the University of New Mexico.

Shahinpoor anticipates that each disposable sensor will cost between 20 and 30 cents. In addition, he foresees a reusable version that consumers could mount in their home freezers, as a warning device should the power go out and the contents thaw. It would cost less than $10, he said.

The sensor uses a short wire that changes its shape when the temperature rises above 32 degrees, or any other predetermined point. The new shape turns a lever, which changes the indicator's color from green to red. No power source is necessary for the device to work.

The wire, which has the diameter of a hair, is made of a "memory alloy" of nickel, titanium and other materials. The alloy has a certain crystalline structure that allows it to "remember" shapes. When the temperature changes, the crystals go from one cubic arrangement to another. The change in the crystalline state causes the materials to pull together, and the wire contracts, Shahinpoor said.

Sandia first developed the system to determine extreme temperatures inside missile silos.