How pure is water? Is there stuff in there that we don't see? We will see if we can trick it.

We have two small jars of water. We also have some table sugar and table salt. Let's look at the sugar and the salt with a hand lens, examine both of the substances carefully and remember what they looked like.

Put a teaspoonful of salt into one jar and a teaspoonful of sugar into the other jar. The sugar and the salt will dissolve in the water. Then we'll put them by a sunny window or a heat source, and allow the water to evaporate. In a few days, we'll look at the residue in each jar, with our hand lens, of course.

What do we see? Does the salt look like it did before we put it into the water? Does the sugar look like it did before we dissolved it?

Looks like there's some other stuff in the water. I wonder if that means that water sometimes (or maybe always?) has undissolved solids in it that we don't see.

We might be able to find stuff in water by just putting some into little jars or bowls, to evaporate, and then see if there is residue that we can examine with our hand lens.
Maybe there are undissolved solids in the water that are so small we can't see them even with the hand lens.

It's a mystery!