Christine Geary, age 12, of Calgary, Alberta, Canada, for her question:

IS THERE SCIENTIFIC PROOF THAT UFO'S EXIST?

An unidentified flying object, called a UFO, is a strange light or object that can sometimes be seen in the air or near the ground. A UFO has no known cause.

Some say that UFOs may be spaceships from outer space. Others say that they have natural causes, even though scientists cannot explain all UFO reports.

Scientists have provided logical explanations for many UFO reports.

Some have turned out to be meteors, planets, rockets, stars, artificial satellites or weather balloons. Aircraft or their exhaust trails, seen under unusual lighting conditions, have also been reported as UFOs. In addition, atmospheric conditions may produce optical illusions that are described by some as UFOs.

Scientific study has not been able to explain all UFO reports. But in addition, there has been absolutely no proof that any UFO came from another planet.