Educate your elementary scientists about the process of evaporation. Introduce the five stages of the water cycle to your students: condensation, precipitation, infiltration, runoff and evaporation. Turn the water cycle into a classroom task by splitting your students into five groups and asking each group to design posters for each stage of the cycle. Pin these posters on the classroom display board in the correct order and explain the poster contents to the rest of the elementary class as you pin it on the wall. Focus, in particular, upon evaporation before introducing an evaporation-specific experiment to your young scientists.
Inform your elementary-schoolers about the experiment that you are going to carry out, outlining the experiment condition, where you will set up two identical containers of water next to one another, but leave one container covered with plastic wrap and the other in open air. Have your elementary students write down their predictions for the experiment, with one sentence dedicated to each container in the experiment. As this is elementary-level science, your youngsters do not have to explain why they have chosen their hypothesis except at a very basic level.
Set up your experiment using two identical transparent glass or plastic containers that have a measuring scale up the side. Fill each container three-quarters full so the water level is at the same height in each, and wrap one sheet of plastic wrap over the top of one container. Secure this in place with masking tape or an elastic band, if necessary. Leave the two containers somewhere in the classroom where they will not be disturbed. Gather elementary students once a day, preferably at the same time of day, to note the water level in each container. Continue your evaporation experiment for at least five days so you get enough results to draw a graph of your results.
Educate your elementary students about the best ways of setting up a science fair display board, including the best way to graph and display results. Teach your young scientists about plotting a graph, leading students through the graph you can display for this experiment. Show your students how to plot a line graph to show the water level, with time in increments of single days along the horizontal and water level on the vertical scale. Students will note that the water in the uncovered container will evaporate while the water in the covered container will condense on the underside of the plastic wrap before dripping back into the water, replicating a mini water cycle within the container.