Awesome primary school science experiment
Pupil fills cup with tap water with ordinary table salt dissolved in.
Takes string attached to a lolly stick and adds paper clip for weight to other end so it stays in the brine when dropped in.
Leave for a few days. Salt laden water creeps up the string, then evaporates, leaving the salt behind.
This is the result! So much in this lesson - solutions, gravity, evaporation, deposits - love it. No 3 daughter liked it so much at school she repeated for us at home.