Take a large bowl or bucket of water and add stones, pebbles/shells and sand (as if you are creating a beach). Mix it up well.
Task/Explore- How can you separate the materials in the water?
(If you have them, try using different sized colanders, sieves, strainers or slotted spoons)
Challenge- add salt to the water. How can you separate the salt and the water? This is KS2 science!
(Clue- put a small amount in a saucer and put it in a sunny place. When the water evaporates, it will leave salt on the saucer.)