How can you separate sand from salt?

Picture a little pile of sand on the table, with a spoonful of salt spilled right into it. Run your fingers through it and feel the grains crunch and slide. The white salt and the golden sand are jumbled up together.

Picking out every grain of salt by hand would take all night long. So here is a puzzle for us to wonder about. How can you separate sand from salt?

First, you pour the whole mix into a glass jar and add some water. Then you stir it with a spoon, round and round. Stirring helps the water touch every single grain.

As you stir, the salt grains shrink smaller and smaller until you cannot see them at all. The salt dissolves, which means it breaks into pieces too small to see and spreads through the water. The salt is still there, hiding in the water.

Sand is made of tiny bits of rock, and water cannot break them apart. So the sand does not dissolve. It sinks and rests on the bottom of the jar.

Next, you need a filter, which is a piece of thin paper with holes so small that water slips through but sand gets stuck. You set the filter in a funnel over an empty cup. Slowly, you pour everything from the jar into it.

The salty water drips through the holes into the cup, carrying the hidden salt along. The sand grains are too big for the holes, so the filter catches them. Now the sand waits in the filter, but how could we get the salt back out of that water?

You set the cup of salty water on a sunny windowsill and let it rest. The warm sunshine makes the water evaporate, which means it turns into bits too small to see and floats up into the air. Bit by bit, the water drifts away.

Each day the water sinks lower in the cup as more of it evaporates. The salt cannot evaporate, so it stays behind. When the water is gone, white grains of salt sparkle on the bottom of the cup.

Dissolve, filter, evaporate. You stir the mix in water so the salt dissolves, pour it through a filter to catch the sand, and let the sun make the water evaporate so the salt stays behind.

Now you know how water, a paper filter and some sunshine can pull two mixed-up things apart, so rest easy and dream of sparkling grains.