Why does a rock feel lighter in water?

Yes, it's buoyancy. An object submerged in a fluid experiences an upward force equal to the weight of the volume of fluid displaced. Whenever you put the rock in the water, all the water on top of it and to the side of it is trying to fall underneath it. This exerts a small upward force on the rock, which fights against gravity's pull on the rock. Since the rock is heavier than the amount of water that would fit into the same space, it does fall anyway, it just isn't as heavy. The same thing is actually happening in air too! If you tried to lift the rock in a vacuum, it would feel heavier.

Actually, the answer IS buoyancy. Buoyancy is the upward force that is exerted by a dense fluid on any object that is immersed in the fluid.

http://en.wikipedia.org/wiki/Buoyancy

If the object weighs less than the buoyant force, then the object will float. Othewise, it will just feel lighter.

Actually the rocks weight feels lighter in water because the sensation you feel is the Buoyancy of your hand holding the Rock. your hand has a greater surface area

no gravity