One of the fun things you can do in Boulder is to take a ride in a Glider plane. They do not have engines. They glide! Its really quite an elegant site and when they are just overhead, all you hear is a swoosh noise.
The way they get up is by being pulled (you can see the tow rope on the front of this one) by a prop plane.
My son loves this. Its a regular sight around here to see the prop plane pulling a glider in the air and then to see the 'release' as the rope is dropped and the glider is left to its own accord.
This morning, my son asked if we could tie a piece of string between his two toy planes and go over to the airport to watch them taking off and landing. So we packed our lunch, parked and sat on the hood.