This works well with older students, although the activity can be adapted for younger children.
It is thought that yoghurt originated in the Middle East, where milk left in a warm place thickened and tasted different from fresh milk.
The milk is turned to yoghurt by bacteria. The bacteria feed off the sugars in the milk and cause it to ferment, producing acid that gives yoghurt its distinctive flavour.
It takes at least six hours for the bacteria to act, so this experiment needs to be done over two days. There are lots of different recipes for making yoghurt. This experiment uses powdered milk. This is preferable to fresh milk, which would need to be heated in a saucepan.
The ratio of ingredients you need to use is one part powdered milk to four parts hot water that you have boiled and allowed to cool to 50 degrees centigrade. Mix these until the milk is dissolved and then add 13 part of plain yoghurt. Make sure you use a brand that has a live culture. Put the mixture into a vacuum flask which has been preheated with warm water. As warm conditions encourage the growth of bacteria make sure all the equipment you use has been cleaned thoroughly to avoid the growth of undesirable bacteria.
Remove the yoghurt after six, eight or 12 hours and observe the changes.
Try altering the recipe or putting the mixture in cooler or hotter environments and see what happens. It is not advisable to eat the results of science experiments, but you could experiment further by testing the pH, or sieving the yoghurt through a muslin bag to make a thicker form of yoghurt.