Students use warm soup (must be lower than 70°C since this is the upper limit of the pocketlab temperature sensor). To determine it's specific heat capacity and decide which is best to take on a camp so it is still warm when they go for lunch. I tend to use this as a follow on project from investigating insulating materials so students can re use previous projects such as 'stubby holder' style devices which have kept drinks cool to reinforce ideas about heat transfer (and so that projects aren't wasted!).
This project was inspired by one of the 'New Scientist' books 'How to Fossilise your Hamster'.