Third-graders are old enough to work in groups without intense supervision, but still young enough that sitting still for too long is a chore. Encourage creativity and take advantage of their group skills by giving your third-graders hands-on projects, like building a cardboard harp. Homemade harps work well as a follow-up to a music lesson or history lesson. Different ancient societies had different ways of making harps than modern luthiers. Drive this concept home for your students by helping them make an aeolian version of the instrument.
Open the bottom flaps of your cereal box and cut up one corner to open up the box. Spread the box out so you can see the plain side.
Draw an oval on one of the big panels of your box. Leave about 4 inches between the top and bottom of the panel and the ends of your oval. Also, leave about 3 inches between either side of the panel and the sides of the oval.
Poke a hole in the centre of the oval with scissors, and cut to the edge of the oval. Angle the scissors around the sides of the oval to cut it out. This creates the hole for your harp's resonance chamber.
Fold your box back into a box-shape with the plain side of the box facing out, i.e. put it back together inside-out. Tape the box closed along the edges and ends with masking tape.
Paint the box as desired. You can use designs similar to those used on ancient harps or create your own designs and symbols. Let the box dry for at least an hour. Paint two unsharpened pencils while the box dries. Let them dry for an hour as well.
Glue each pencil horizontally to either end of the box, positioning them about 2 inches away from the ends of your oval. Let the glue dry for about 10 minutes.
Push six brass fasteners into each end of the box, spacing the fasteners about 1/2 inch apart and about 1 inch from your pencil. These will be the "tuning keys" for your harp.
Cut six pieces of string about 15 inches long each. Wrap one end of each string around a brass fastener on the left end of your box.
Bring the string down to the corresponding brass fastener at the right end of the. Wrap it around that fastener so the string is taut over the two pencils and the oval opening. Pluck the strings to play your harp.