# How do you explain straightness to a 5-year-old?

How do you explain straightness to a 5-year-old? I am having a hard time trying to explain straightness to little kids. I have been asked, "How do you know that the ruler is straight?" Are there any physical activities that can be done to demonstrate that something is straight?

I have done this with my five year old: If you put two points on a piece of paper and ask them to draw the shortest path from one point to the other. It's good fun watching them work out it out, but for her, at least, she got the idea pretty easily.

You can define a straight line as something that looks the same after you flip it (either parallel to itself or perpendicular to itself), rotate it, or move it along itself.

To see if an edge of a ruler is straight, first draw a line with it. Then flip it over (one way) and draw a new line over the old one. Do it again different ways (flip over the other way, rotate, move along itself). If they look different, then the ruler isn't straight.

• You could also try making a fold along the drawn line. That would make sure you didn't accidentally draw non-straight parallel curves. en.wikipedia.org/wiki/Parallel_curve