Equipment required (per set)
The mixture should be mixed slowly to remove any lumps. You may need to add a small amount of water or powder to get the right consistency. You will know when you have got there when if you try to mix it fast, it 'rucks' up at the edges whereas if you mix slowly, it looks more like a liquid.
The substance is a mixture of solid particles (powder) suspended in a liquid (water), this is called a 'colloid'. When you stir the mixture slowly, the particles move around in the water freely and so the mixture behaves like a liquid. When you mix more thoroughly the solid particles rub together causing friction which makes them stick together making the mixture behave more like a solid.
Students love playing with the substance so it may be worth giving each a tray to place the bowl in. Any remains can be flushed down the sink with lots of water. Avoid pouring thick mixtures down the sink before watering them down as the mixture may settle in the 'u' bend.
The TV programme 'Braniac' famously featured a man walking across the surface of a custard filled swimming pool. Although in that clip the pool was not as deep as it seemed, this is certainly possible. When weight is quickly added to the surface, the particles beneath rub together and form a kind of solid column for just long enough to take the weight. There are likely to be lots of health and safety implications if you attempted filling a swimming pool in school, so don't try it!