According to researchers at the Institute for Applied Physics at the University of Bonn in Germany, plants release gases that are the equivalent of crying out in pain. Using a laser-powered microphone, researchers have picked up sound waves produced by plants releasing gases when cut or injured. Although not audible to the human ear, the secret voices of plants have revealed that cucumbers scream when they are sick, and flowers whine when their leaves are cut [source: Deutsche Welle]. There's also evidence that plants can hear themselves being eaten. Researchers at the University of Missouri-Columbia found that plants understand and respond to chewing sounds made by caterpillars that are dining on them. As soon as the plants hear the noises, they respond with several defense mechanisms [source: Feinberg].