start

Click the start button to begin the boxes flashing at slightly different intervals.

The left box changes to grey for .5 seconds, changes to white for .5 seconds, and then changes back to grey.

The right box changes to grey for .5 seconds, changes to white for .6 seconds, and then changes back to grey.

This extra .1 second of white on the right box makes the two boxes flash grey at different intervals.

Notice how they get increasingly out of sync until the flashing is perfectly opposed - when one is flashing grey, the other is flashing white. Then they gradually return to sync - flashing grey simultaneously one time before going out of sync again. The whole cycle repeats every 11 seconds.