Put a bunch of gas particles in a container and set them loose in random directions. The more you add, the more likely you are to see them evenly distributed between the two sides of the container. Simply put, there are more ways to rearrange them if you split them (roughly) evenly. The logarithm of the number of possible combinations is called the entropy. Physical systems like to be in states where they have a lot of choices—in other words, they prefer high entropy states. This is one of the principles that makes the initial conditions of the inflationary picture so incredibly unlikely.

The demonstration below illustrates this. To use this demonstration, click the “Open Controls” button in the top right, which will allow you to add and remove particles, change their speed, and enable or disable Maxwell’s demon. Notice that as you add more particles to the volume, the more evenly they will be distributed. With a small number of particles, it’s reasonably likely that ALL the particles will be on one side, whereas this probability is extremely small with more and more particles.