M.I.T. researchers who more typically work on new ways to detect chemical weapons and explosives have created an affordable hand-held sensor that can tell when fruit is ripening.

Techniques already exist to detect ethylene, a hormone that initiates ripening, in the air around fruit, but they are expensive and unwieldy. The new sensor, which need not even touch the fruit, includes tens of thousands of carbon nanotubes, with a copper compound added, and might cost as little as a dollar (including a chip that would transmit information...