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?
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.
For a ruler: hold it up, perpendicular to your face, adjust it so the entire edge of the ruler seems to be a single point. If some parts of the edge bulge this way or that, you will be able to see that it is not straight. (Of course this pushes the question to something else: does light travel in a straight line...)
I best do this by placing any object at the centre of a neatly drawn (perfect) circle and tell them that the relative distance between the object and any point on the circle is straight. So if it is a ruler, I keep one end fixed and rotate the other edge about the fixed point to draw a circle and the tell them that any distance the ruler creates from centre to circumference of circle qualifies as STRAIGHT.