Androids are robots whose outer appearance resembles that of a human being. Although such automatons have not yet been made on Earth, extraterrestrial civilisations may already have utilised them to operate interstellar spacecraft, since they would not be limited by short life spans. In addition, androids would not experience the physical stress which would affect humanoids when executing sharp turns at high speed. Some witnesses, as in the Falkville, Alabama case, have claimed that they were unable to determine whether the alleged UFO occupants were natural biological beings or robots. It is possible that UFOnauts described in such reports were, in fact, androids.