Have you tried this thought experiment to investigate the water cycle? "‘Tagging’ water molecules – to explore the water cycle" is one of a series of ELIs about the water cycle.
Pupils carry out a thought experiment to visualise the movement of an imaginary ‘tagged’ bright blue water molecule as it moves through various parts of the water cycle. Molecules can actually be ‘tagged’ and traced using radioactive isotopes, so the principle is used – if not the bright blue colour.
There are five Earthlearningidea Water Cycle activities and they demonstrate pupils' progression of thinking skills. They can be found in the index on the website under 'Water cycle'.