A simple practical aimed at helping students to make a prediction. Pour 20mL of water into a 100mL measuring cylinder, add a squirt of washing up liquid and ¼ of a Berocca tablet, (any effervescent tablet will work – I just used this one as it doesn’t present any health and safety issues). I provided each group with a bag of Berocca tablets already broken into quarters, so they had to work out how many ¼ s made a ½ etc. It is a good idea to do this in a tray as it may overflow – although usually it runs out of water first, which is another good talking point.