Hummingbirds are birds that comprise the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.5–13 cm (3–5 in) range. Indeed, the smallest extant bird species is a hummingbird, the 5-cm Bee Hummingbird. They hover in mid-air by rapidly flapping their wings 12–80 times per second (depending on the species). They are known as hummingbirds because of the humming sound created by their beating wings, which sometimes sounds like bees or other insects. To conserve energy while they sleep or when food is scarce, they have the ability to go into a hibernation-like state (torpor) where their metabolic rate is slowed to 1/15th of its normal rate. When the nights get colder, their body temperature can drop significantly and thus slow down their heart and breathing rate, thus burning much less energy overnight. As the day heats back up, the hummingbirds' body temperature will come back up and they resume their normal activity. They can fly at speeds exceeding 15 m/s (54 km/h; 34 mph) they are also the only group of birds with the ability to fly backwards. Individuals from some species of hummingbirds weigh less than a penny.