"There are two neural events that we believe are hallmarks of the 'aha!' moment for the rat," says Linda Hermer-Vazquez. These are high-frequency activity in one subset of neurons, and decreased activity in two other areas, she says.

"Artificial noses don't work well when there are other smells around," says Christiane Linster, an olfaction expert at Cornell University in New York. "Rats are good at that."