Find two spots - a sunny area and a shady area of concrete
Dissolve some salt in a large measuring jug of water
Pour an equal amount of water onto the concrete forming two big puddles one in the sun and the other in the shade. Trace a line around the outside of each puddle
Dissolve some more salt in some water and pour an equal amount of this into each bowl. Then place a bowl of salted water next to each puddle
Measure across both the puddles with the tape measure and then measure the depth of the water in both the bowls with the plastic ruler and record the four measurements
Return to the area every hour and keep measuring the puddles. You can keep drawing new lines around the puddles as they get smaller. Also check the jars each day to see what happens to the water and salt.