This is probably one of the simplest activities you can set up for your children, though it can get pretty messy.
C found the instructions in his book "50 Science Things to Make and Do", a Christmas gift from one of his aunties.
All you need is a large bowl (washing up bowl is great), aprons, cornflour and a little food colouring (optional).
C followed the instructions to make the gloop himself.
Pour two cups of cornflour into the bowl (this was the messiest part in the inkspots house, somehow my children had forgotten how to pour and there was cornflour everywhere, but a dustpan and brush soon put this to rights).
Add a cup of water and a few drops of food colouring.
Mix together with hands.
Once mixed gloop has some very odd characteristics. It is made of long thin molecules particles which don't dissolve in water. When you apply pressure or roll the gloop the particles join together and the gloop acts and feels like a solid. When allowed to dribble or rest the particles slide over one another and the gloop acts and feels like a liquid.
The children remained engrossed in this weird liquid/solid material for almost an hour, squeezing it, punching it, pouring it, rolling it and then dribbling it through their fingers.
It did splatter across the table, but then goes back to solid form, making it pretty easy to sweep away afterwards. At the end don't wash down the sink, as it may cause a blockage: either place in your kitchen bin, or allow to dry out to a fine powder, which can be used as a sensory material soil for toy diggers, or can later be rehydrated to make gloop again.