Find the regularity

Describe the patterns in the following examples. What is the reason for these patterns?

Footprints in the sand. Source: Ajith Kumar. Licensed under a Creative-Commons Attribution 2.0 Generic (CC BY 2.0).

A ladder.

Jane made the following drawing and said there was a pattern in it. But her teacher could not see the pattern. Can you?

Jane’s drawing.

When you have finished puzzling over these questions, look at some explanations in Explaining Regularities.