Engineering Design Hydraulics & Pnuematics
Two states of matter - liquids (hydraulics) and gases (pneumatics) 
Hydraulics - the means of transferring (and altering) applied force by the medium of a fluid, fluids being incompressible. Crucial design rests within the mechanisms of transfer, especially the rams and sealing technology applied. Not every application has 'killer' requirements, and thus the exploitation of the principle comes in very handy in product design. Above (in green) is a linear gate valve for controlling flow in the petrochemical industry.
Pneumatics - the (usually) mechancial actuation by means of using/exploiting pressurised gas (such as air). Common use for industry are linear actuators to effect mechanical motion, usually in combination with an ECU for precise timing. Again, very handy to exploit the pricnciple in product design, machine design or electromechanical design.