They were asked to mix together marbles, salt, sand, rice and water. They eagerly did this, but were then asked to separate all the ingredients back again. Although initially stumped, the children worked in their houses to figure out what they needed to do. The marbles came out easily with a spoon and they realised that a sieve was perfect for the rice. Filtering the remnants ended up with clear salty water and they have worked out that they need to evaporate the water as the final step.