Fill a wide-mouthed jar halfway with water.
Add non-iodized salt to the water using the spoon, one spoonful at a time. Stir the water between adding spoonfuls to dissolve the salt. Continue adding salt until the 1/2 cup is added, or until the salt stops dissolving, and accumulates at the bottom of the jar.
Tie a strong piece of string securely around the middle of the pencil. Balance the pencil vertically across the top of the jar. The string should hang directly above the surface of the water. Trim the string with scissors to shorten the length if necessary.
Leave the jar undisturbed for three weeks. Have students monitor and record the growth of salt crystals as they gather on the string and how the water level changes as the crystals grow.