Determine the shape you want your bridge to take. It could take the simplest form of an arch like the Harbour Bridge in Sydney, or a suspension bridge such as the Brooklyn Bridge in New York City. Some assignments require you to make specific bridges of certain lengths and widths. Others may require you to use a certain number of toothpicks, balance a certain weight and join toothpicks of different heights and widths to achieve a specific shape.
The arch bridge is the easiest to design because its shape requires fewer mathematical computations. Scale and draw the bridge on a graph paper to guide you on the required length. Reinforce your building material for the bridge to withstand weight and to make the toothpicks level. Join toothpicks together so that the pointed end of one toothpick is glued to the blunt end of the other to lay a foundation for the arches.
Use pins to hold the intersections between the adjoining toothpicks together to prevent the arch from collapsing. These hold the joints together as the glue dries. When the glue dries, hold the arches up onto a foamcore board and pin them at their bases. Arrange them so that two arches form one side of the bridge.
Reinforce the sides of the bridge to make it stable by joining pieces of toothpick horizontally onto each side of the arches. Make supporting pillars to hold the arches from the top and onto the foamcore. Select toothpicks with a blunt base and glue them together in the shape of a bundle. Depending on the position of the foamcore, vary the sizes and glue them in place to support the arches. These are posts for the bridge. When the glue on the posts dries, finish your project by gluing more toothpicks across the two sides of the bridge.