Replace an action in your system with an action of the opposite sort.

Illustration:

As ballast for the bathyscaph (a navigable ocean-diving vessel designed to reach great depths), inventor Auguste Piccard placed steel shot in steel containers fitted with electromagnetic valves. Each valve consisted of a cylindrical pipe around which an electric magnet was wound. When current was flowing, the shot coalesced and acted as a reliable plug. By varying the current, the shot discharge could be precisely controlled. If the current was interrupted for some reason, the shot was discharged and the bathyscaph surfaced.

As an emergency provision, the shot containers were held in place by electromagnets, which discharged from the bathyscaph if current was interrupted. These electromagnets were continually charged by a battery. If all jettison devices failed, the charge died out within 16 hours, the containers separated from the bathyscaph, and the bathyscaph surfaced.

Replace a sequence of operations

Consider replacing a sequence of operations with the reverse sequence.

Illustration:

When filming a motion picture, it was necessary to depict an actor emerging completely dry from a lake.

The reverse-filming technique was used: the camera was run in reverse, while the actor, wearing dry clothes, stepped backward into the lake. When the film was played back (in the usual direction), the desired effect was achieved.

Reverse filming is similarly used when filming dangerous sequences, such as depicting a person being hit by a fast-moving car.  To film this stunt safely, the actor lies down in front of the car; the camera is then run in reverse as the car backs away and the actor stands up. To achieve a more realistic visual effect, reverse filming is carried out at a slower speed.

Inside-out or upside-down

Turn the system inside-out or upside-down.

Illustration:

Forms for casting artificial limbs can be molded using sculptors, but sculpting labor is expensive.

A less expensive approach involves forming an elastic copy of a normal limb by coating the limb with resilient material. The elastic copy can then be turned "inside out" to create a form for casting the artificial limb.

Replace external action with internal

Substitute an external action for an internal one (or vice versa)

Illustration:

Restaurant workers have to uncork many bottles.

A pressure gun can make uncorking more efficient. A sharp hollow needle at the end of the gun is pushed through the cork. Gas under pressure from the gun then forces the cork out of the bottle.

Instead of heating use cooling

Replace an action in the system with an opposite action.

Illustration:

When hot water is poured on frozen ground, the ground is defrosted but it also becomes muddy.

The mud can be prevented. If the frozen ground is surrounded by a low, waterproof fence and cold water is poured in, the water freezes. As the water changes to ice, heat is given off and the ground is warmed. A day or two later, the ice layer can be removed with an excavating machine and the unfrozen ground can be easily worked. Inclined, as well as level sites can be defrosted by this technique.