Determine the goal of your science project. Do you want the children to learn how quickly water evaporates? To learn to test their theories about what will happen? To learn about what makes liquid turn to gas? Once you decide your primary goal, you can create a project tailored to that goal.
Evaporation can occur rapidly or slowly. If you want the students to learn several means of evaporation, consider choosing a few brief projects. If you want the students to learn to theorize possible outcomes, consider longer-term projects. Sample projects for fast evaporation are: boiling water in a teapot or Bunsen burner and measuring how much boils away; and putting one wet sponge in front of an electric heater, another in front of a fan and a third in a location away from both. Sample longer-term projects are: putting a water-filled aquarium covered with plastic wrap in a window, and measuring each day how much the water level goes down; and putting two or three wet washcloths in different locations, with one lying flat, another crumpled up and a third exposed to wind. These projects measure the rate of evaporation and the effect of heat and wind.
Gather the needed materials and supplies. Effective projects can be done with measuring cups and plastic wrap, or you may require more complex materials like chemistry labs and burners. Some outdoor locations with a breeze may also be necessary.
Some science projects are best done in teams. The team should theorize together what will happen, for example which washcloth will dry most quickly. Working in teams can make the science project more engaging, and children will teach each other as well.