Scales are used
to weigh objects.

This kind of scale
is a special one
called a balance.

After placing the "rock" in the pan, you move the largest weight on the balance to the right, until it begins to weigh the balance down below the zero marker. Leave the weight in the little indented spot which is closest to, but not after, where it started to go down.

Then, keep moving the
smaller weights until the
two sides are balanced.

Or, if you are done making observations,
you can continue with your investigation...