Upthrust

If you try to lift up a weight in a swimming pool and then try to lift the same weight on the edge of the pool, it feels much lighter in the water.

This was supposed to have been first explained by the Greek scientist Archimedes. He said that the water gives an upward force or upthrust on any object in it.

You can weigh an object in air and then in water and actually work out the upthrust, it is the difference between the two readings. For this reason the upthrust is often called the loss in weight of the object.

Here is a video that I think explains this principle well;