Building a bridge out of cardboard can teach children basic engineering techniques. The great suspension bridges that span gorges, rivers and railways can be created from a few simple supplies. After building the cardboard bridge, explore these types of bridges further by finding photographs of famous suspension bridges, including the the Clifton Suspension Bridge, the Humber Bridge and London's Tower Bridge.
Cut a sheet of poster board to measure 30 cm (12 inches) square for the bridge base.
Cut two paper towel tubes in half for the bridge anchorages, which are towers that are about half the size of the bridge's main towers and help balance the suspension bridge. Attach the tubes in pairs upright to the base with masking tape at opposite ends of the base, 10 cm (4 inches) apart and facing each other.
Attach two pairs of paper towel tubes, upright, directly in front of the anchorages with the masking tape. Cover each anchorage and tower top with a glued-on circle cut from poster board slightly larger than the tube diameter. Spray paint each anchorage and the tower grey. Spray the base with the blue spray paint, using short spurts of paint, to represent water for the bridge base, covering up the masking tape.
For the bridge cables, cut two pieces of string to measure 37.5 cm (15 inches). Glue one string end to the top of an anchorage. Bring the string up and glue it to the top of the tower. Connect the string to the opposite tower and anchorage with glue, letting the string drape down between the towers to approximately half the tower's height. Repeat for the opposite parallel cable.
Cut a piece of poster board to measure 30 cm (12 inches) long and 7.5 cm (3 inches) wide for the bridge deck or roadway. Spray paint the deck grey.
Beginning at the middle of the suspension bridge, attach pieces of wire to the cable string. Bend the end of each wire to attach to one side cable, curve the wire under the bridge deck, bring the wire back up to the opposite cable and attach by bending.
Cut each piece of wire to fit, making sure the deck remains straight. Attach approximately six pieces of wire between the tall towers and two more between the towers and the anchorages to securely suspend the bridge deck or roadway.