Quite how cats manage to rapidly alter their weight, while keeping their mass constant, is beyond science to explain. It is probably encoded in the same gene that allows them to walk through walls.

Although it remains a mystery, this phenomon should be harnessed for the good of man and cat kind.

Here is my idea for an aplication, use cats as variable weight ballast for transportation purposes:

A train requires greatest traction when accelerating or braking hard but can still manage high speeds with less weight.

A hot air balloon needs ballast as does a ship.

The problem is how to persuade the cat to alter its weight on demand. I believe that a simple mechanism, which I will describe below, would make a cat alternate between very heavy and very light states.

The mechanism of which I speak consists of a recumbant human in a bed, upon whose chest a cat sits. The second part of the device is a high shelf on which sits some cat treats. The shelf can be hidden or revealed to the cat by a flap.

With the flap closed the cat will revert to its heavy state providing balast. When the flap is opened, the cat spies the treats and prepares to jump causing it to switch to the light state.

Thus the weight state of the cat can be controlled on a whim.

There may be some latency in the system on closing the flap as the cat will not immediatly return to dozing on the humans chest.