There is a fascination with sticky liquids that is common to most children and not a few grown-ups. It can be the slow-motion drip of chocolate sauce, the glutinous oozing of golden syrup, or even the more prosaic spreading of engine oil.
Viscosity is shown by all fluids, including water. It is a kind of internal friction. Liquids tend to stick to solid surfaces. This means that a liquid flowing through a pipe or across a river bed is stationary where it touches the solid. Children will be surprised to hear this. There are some fun ways to investigate the viscosity of liquids. The fact that you can eat some of the spare materials is a bonus.
* The really sticky race
Take a metal baking tray and place large blobs of viscous materials along one edge. Try chocolate sauce, glycerine, shampoo and syrup. From their knowledge of these materials, ask the children to predict the winners.
Raise the tray to about 45 degrees on a block and follow the progress of the crawling fluids.
* Extend the investigation by warming the sticky fluids or by cooling them in the fridge.
* Depth charges
One way you can compare the viscosity of liquids is to drop a marble or steel ball-bearing into the liquid. The more viscous the liquid, the longer it takes to fall. Use a tall narrow jar filled with the first liquid, say shampoo. The advantage of using a ball-bearing is that you can use a magnet to retrieve it. Place two elastic bands around the jar, about 15 or 20cm apart. Time the falling sphere as it travels between the marks.
Follow up with a discussion about why winter engine oil is more runny than the summer oil.
Ray Oliver teaches science in Hertfordshire
Sophie Duncan is back in Science Corner on September 19
Science Subject Focus, pages 22-29