For those who love to keep leftovers, and other food items for months, and sometimes years, this gel will tell you if it is safe to eat. It is actually a nanorod-embedded smart tag that changes from red to green, and it mimics the growth of bacteria in milk without touching it.

Developed by Chinese scientists, the gel with tiny gold and silver nanorods make use of some vitamin C, acetic acid, lactic acid, and agar.

"The gold nanorods are naturally red," explains CBS News, "and over time, the other compounds like silver gradually deposit onto the gold nanorods, forming a silver shell layer that alters the shape and composition of the nanorods, a process that ultimately changes their color."

It also responds to temperature, and turns green faster in conditions where bacteria might grow faster.

[American Chemical Society via CBS News]