Liquids and gases are both considered fluids for certain purposes of analysis. Fluids are substances whose molecules, unlike those of solids, do not occupy fixed positions at normal temperatures because the forces that bind them together are relatively weak. From experiments conducted in the eighteenth century, the Swiss mathematician Daniel Bernoulli determined that for a fluid in motion, pressure and velocity are inversely related: pressure is greatest when speed is lowest, and vice versa.

By applying Bernoulli's principle, aeronautic engineers have been able to design airplane wings that can help put 400-ton passenger jets in flight and keep them there. The top surface of the wing is curved and the lower surface is flat, and as a result, the air rushing over the wing, which has a longer distance to travel, has a greater velocity than the air passing under the wing. Because pressure is greatest where velocity is least, the pressure pushing up on the wing from below is greater than that pushing down from above. The difference in pressure provides a net upward force, called lift, on the wing.

We can apply Bernoulli's principle to explain why a ping pong ball in an upside-down funnel can be made to seemingly defy gravity, as in this demonstration adapted from ZOOM. The air blown down through the funnel moves around the surface of the ball, creating a lower-pressure area above the ball, where velocity is greater, and a higher-pressure area below the ball, where velocity is lower. The net effect of this pressure difference is upward air pressure on the ball, which prevents it from dropping out of the funnel.