How would you explain density to a fifth grader?

Answer 1

In a well-known but probably apocryphal tale, Archimedes was given the task of determining whether King Hiero's goldsmith was embezzling gold during the manufacture of a golden wreath dedicated to the gods and replacing it with another, cheaper alloy (lead wrapped in gold).

Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated easily and compared with the mass; but the king did not approve of this. Baffled, Archimedes took a relaxing immersion bath and observed from the rise of the water upon entering that he could calculate the volume of the gold wreath through the displacement of the water.

To a fifth grader show this in practise, put same sized and weighted objects in a bowl of water and measure the overflow with kitchen scale. Then ask why similar objects made of different materials displace different amount of water.

You can also show him/her a diagram of a 'bottle of molecules' in the first picture draw big cirles (all equal size!) loosely filling the space. Next to it draw a tight-packed bottle, with small circles (molecules) crowded, filling the space.

Now explain the first bottle is a low-density object and the second high-density object. She might ask why is the first bottle same in weight as the second one. Then explain to her: "yes there is fewer molecules in the first bottle, but these molecules are heavier". Then tell her a low-density object is easier to puncture with needle. Like for example a piece of balsa wood compared to oak.

Because "there is more room between the molecules" For the same reason the dense object displaces more water, because there is no room between the molecules to hold water, so the water has to go somewhere. Low density object 'sucks in' some of the water, so it does not overflow that much.

(this is not actually scientifically correct, but easier to understand for a kid).