Fluids are liquids or gases, which means that the different particles that make up fluids are constantly bouncing about in a highly random motion. A particle is "the atoms or molecules (for example water molecules) that make up the liquid". Since these atoms and molecules are constantly moving in quick, random motions, the particles are always bouncing off of one another.

Check out the following YouTube video for a simulation of how the atoms in a liquid bounce around randomly. The video depicts a simulation of extremely low temperature liquid Argon, which is a gas at room temperatures. Notice that the motion of the blue ball, which is a specific atom that has been highlighted, shows how random and "bumpy" the liquid motion is.

The pressure on the walls of a container results from the same random jostling of the fluid particles that cause the blue atom to move around in the video. At each moment in time, a pressurized container (for example an inflated balloon) has many more collisions occurring from the pressurized gas on the inside than from the lower pressure ambient atmosphere on the outside. This difference in numbers of collisions results in the pressure that you feel on the balloon's surface, which resists compression when you try to squeeze the balloon.

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