At about 10:00AM I was in our backyard with my wife and my 21 year-old daughter. We live about four miles south of Hill Air Force Base. Hill conducts training fly-overs all the time, so seeing aircraft is commonplace. On this particular day an F-16 flew over drawing my attention. Thinking that I might see another, I looked to the north and up and spied a bright silvery object high in the air. It was moving very lazily from north to south and appeared to be bobbing about like a balloon. My daughter also saw it.From time to time Mylar balloons come untied and float away, so I thought what I was seeing was a balloon because object looked round and metallic because it glittered in the sky like silver Mylar balloon.As I watched the object, to my amazement, it suddenly stopped dead, moved a bit further to the south and then did a very tight and quick 180-degree turn and shot off in a direct line to the north, vanishing in under three seconds. I could not tell if it was going up, but it appeared to be, also.For the record, I was raised in Western Washington in the middle of Seattle Boeing Field, SeaTac International Airport, McChord AFB, Renton Boeing field, Paine Field where the 747s are built and at least four regional airports. Simply put, I have seen aircraft of all types. I even saw an SR-71 and a B-2 bomber in flight when they were not supposed to exist. Thus, I think I have a pretty good understanding of how various types of aircraft look on the ground and in the air. I even worked at Boeing field in Seattle for three years.What I saw was not an airplane by our standards. No aircraft that I have ever seen stops in midair and then pulls a turn that, to me, appeared to have a radius of the same length as the object was wide. An airplane would literally have to turn on a wingtip to make such a turn. My wife thought it might have been a helicopter. That is a possibility. Helicopters can stop in midair and turn on their own axis, but the object seemed too high for any helicopter I have been around and definitely moved too fast of one when it disappeared.I wish that I could have taken photos or a video, but the event happened too quickly.