The ruler is introduced in challenge 2 of Coding Adventure Part 1.
To use the ruler:
Locate the ruler on the upper left corner and click on it.
Place the ruler on an object and click.
Move the mouse to the next object, the one you wanted to measure the distance with, and you will see the distance.
You will also meet a life-like ruler in our Dodo Does Math Distances course and Dodo Does Math Hour of Code: