My sister and I were talking to our neighbor around dusk when a UFO appeared behind our neighbor. It was only about 100 ft off the ground and about 250 feet away. My sister and I saw it at the same time. It flew in from above coming toward us and then stopped mid-flight and hovered for 15-20 seconds. It then with instantaneous acceleration darted off in a fairly straight level flight path to our left out of sight. It was very low so we lost sight quickly after it accelerated. The speed when it flew off was very fast and seemingly instantaneous acceleration. There was no sound, no jet or propeller noise, or exhaust from the UFO. Somewhat erie how it could move in stop, hoover and accelerate without any sound of any kind -- it just seemed to be able to jump to an incredible speed after it stopped. What is also interesting is the UFO appeared about a mile NW of Lambert International Airport in the landing path of the landing planes. However, it moved diagonal to the flight path and much lower than the incoming planes. We had lived there for about12 years and were very accustomed to planes coming into land but never saw a plane of any kind where this appeared as it was very low and not even traveling in the direction toward the runways to land. The landing planes were also directly right above our house most to the time. The UFO was also much lower than any plane and the acceleration was incredible from a completely stationary position. My sister and I could not believe what we saw and were mesmerized by the craft. Our neighbor was very startled by how we reacted as we no longer were paying attention to our conversation and staring behind and above her. However, before our neighbor could turn around to look, the UFO flew off and she did not see it. After it darted off, my sister and I looked at each other in disbelief. I have never seen anything like this since nor have I ever seen any aircraft with the flight characteristics of this craft.