4 to 6 books (enough to make 2 stacks the same height) A package of file cards 300 to 400 pennies (loose or in rolls) Scissors

Make 2 stacks of books with a gap of about 4 inches between them. Make sure the stacks are the same height.

Lay one file card over the gap between the books. About 1/2 inch of the card should be resting on a book at each end. How many pennies do you think you can pile on this flat bridge before it falls into the gap-5? 10? 100? Try it and see how close your guess was.

 As an advertising stunt for a paper company, Lev Zetlin Associates designed a full-sized paper bridge that was strong enough to support a car!

Without adding anything to the file card, try to make your bridge stronger. How could you change a file card to make it stiffer? What happens if you fold the card in half? If you make an arch? How about if you fold the card into pleats?

Make a bridge, then test it to see how many pennies it will hold. Some of your bridges may hold a few pennies before falling down. Others may be stronger, but the pennies may slide right off. And some bridges will probably hold a lot more pennies than you'd think.

 How many pennies can my file-card bridge hold? You may find that a file-card bridge can hold more pennies than you'd think! Here are the results of the file-card bridges that the Science-at-Home Team built. A roll of 50 pennies weighs 132 grams-that's a little more than 41/2 ounces. How many kinds of bridges are there? You might think that bridges come in an infinite variety of forms. But if you get right down to the structural elements of a bridge, there are really only three kinds: beam spans, arch spans, and suspension spans. The simplest kind of bridge is a beam bridge. A log that has fallen across a river makes a beam bridge. So does a board laid across a puddle, or a span of steel laid across a body of water, or a file card laid across two books. A beam bridge relies on the stiffness of the building material. If the log across the river sags, it doesn't make a very good bridge. Arches have been common features in buildings since 1,000 B.C., but they didn't appear in bridges for another thousand years. Roman roads, built at the height of the Roman Empire's power, were often supported by stone arches. Suspension bridges, like the Golden Gate Bridge in San Francisco, rely on a cable or rope for their support. Each end of the cable or rope must be anchored to the bank-tied to a tree, a boulder, or (in modern suspension bridges) a massive block of concrete called an anchorage. The cable or rope pulls on the anchors, but as long as they don't move and the cable or rope doesn't snap, the bridge is stable. What kinds of bridges can I make with my file cards? Using just your file card, you can make two of the three different kinds of bridges. When you lay a file card across two books-even if you've folded the card into pleats first-you've made a simple beam bridge. If you cut slots into the card, tuck the flaps under the edges of the book covers, and push the books slightly together, you'll make an arch bridge. We haven't figured out how to make a suspension bridge out of a file card, though. If you come up with a way to do it, please let us know!

 This and dozens of other cool activities are included in the Exploratorium's Science Explorer books, available for purchase from our online store. Published by Owl Books, Henry Holt & Company, New York, 1996 & 1997 ISBN 0-B050-4536 & ISBN 0-8050-4537-6, \$12.95 each © 1998 Exploratorium