This Demonstration shows animations and calculates work for reversible and irreversible expansions and compressions (adiabatic or isothermal) of an ideal diatomic gas in a piston-cylinder system. Select either compression or expansion using the buttons, and compare two processes side-by-side using drop-down menus. Change the final pressure with the slider; the pressure ranges are different for compression and expansion. For adiabatic processes, the final temperature is calculated. For all processes, the final volume is calculated. Press the play button to start compression or expansion. For reversible processes, more weights (blue rectangles) are added to the piston for compression or removed for expansion. For irreversible processes, the external pressure is constant at the final value, so the number of weights is fixed, and initially stops (red triangles) prevent the piston from moving until the play button is pressed. Animations of reversible processes represent slow processes; irreversible processes are much faster and the piston overshoots the final volume, but these behaviors are not shown in the animations.