Water and wetland studies are common in fourth grade, so you can supplement a unit by having students build their own waterfalls out of natural and recycled materials. Send the students on an outdoor scavenger hunt to collect rocks, pine cones, leaves, sticks, straws, plastic bottles and other materials they can use to build the "slide." Let them experiment with constructing different types of waterfall foundations in a plastic tub. Run water onto it with a hose and watch how the water flows. Try different designs to make the water flow in a particular direction or pattern.
Fourth-graders often take field trips to watery habitats to release salmon, plant trees or simply observe the natural environment. Let each student construct her own pond viewer scope to get a better look at the creatures that live underwater. Remove both ends from a tin can, coffee canister or empty tube of chips. Cover one opening with clear plastic wrap and wrap a large rubber band around the edges to secure it in place. Alternately, if your can has a plastic lid, cut the center out of the lid and fasten it over the plastic wrap to hold it in place. Poke the viewer down into the water a few inches and look through the open end to spy on the aquatic creatures living there.
Kites make learning about wind forces and aerodynamics entertaining. The wing shape of a kite affects its flight stability, air flow and buoyancy. The Little Toy Airplane and Airplane and Aircraft websites show a good sampling of many possible kite designs, such as a simple diamond kite, sled kites, delta kites and box kites. Try making several different designs and find a large open area to fly the kites. Compare the flight results and discuss how the structural features of each kite influence its flight capabilities.
Boomerangs are another fascinating way to learn about flight physics in action. Sketch a boomerang shape on a sheet of heavy cardboard and cut it out. NASA provides a four-point boomerang template, while the Society of Women Engineers offers right- and left-handed versions of the traditional arched design. Paint or color as desired. Experiment with different throwing techniques, such as overhand, underhand, on the vertical or horizontal plane and with different hands to get the best return results.
When studies veer toward energy sources, a solar oven turns exploring green energy alternatives into yummy snacks. Cut a three-sided flap in the lid of a pizza box, leaving a 1-inch border around the edge. Line the box and flap with tinfoil on the interior side. Cover the window in the lid securely with plastic wrap. Add a second lining of black paper in the bottom of the box on top of the foil to absorb enough heat to reach a temperature of at least 100 degrees. On a hot, sunny day (more than 80 degrees Fahrenheit), prop the flap open to reflect the solar heat toward the cooking tray and try to cook s'mores in the "oven."