To find out which properties a material has, they have to be tested.
The children tested several different materials to find out more about their properties.
Materials that will dissolve are known as soluble. Materials that won't dissolve are insoluble.
The children found out which materials are soluble and which are insoluble.
Separating Processes - Evaporation
When the salt water solution is evaporated, the water will turn into water vapour and leave the salt behind.
Separating Processes - Filtration
Sand is insoluble, so it has not dissolved in the water. The sand particles will not be able to get through the tiny holes in the filter paper, but the water particles will.
The sand will be caught in the filter paper while the water will get through to the bowl.