Strange Cornflour Slime
Cornflour, as much as you can.
Put most of the cornflour in the bowl
Add water to the cornflour until it starts behaving strangely - you will want at least twice as much cornflour as water, so don't add it too fast.
Try moving your hand through the liquid slowly, then fast, have a play with it.
You end up with a really strange mixture, with when you move slowly it will flow slowly, but if you try and change its shape quickly it goes hard. If you get the consitancy right, you can even roll a ball out of it that will bounce, then when it stops, flow as a liquid again. It is known as a shear thickening liquid.
Cornflour is made up of lots of tiny (<0.01mm) starch particles, these are very attracted to water so the water gets in amongst them very quickly.
The water acts as a lubricant, so when you move it slowly the particles have time to move past each other and they can flow like a liquid.
However if you apply a rapid force it causes the particles to move slightly causing the particles that are almost touching to jam together. and the water that was between them moves sideways slightly into the gaps.
Now instead of having lots of lubricated individual particles you have a solid structure of lumps touching each other which can't flow.