Screws : keep things secure

If you were building a tree house, what could you use to keep the boards together? You could use nails, but you might also use screws. Screws are one of the simple machines that are designed to make different kinds of work easier! Lifting, pulling, pushing, and fastening things together-all these tasks are more efficient when you use a simple machine. In Screws Keep Things Secure, readers learn how a screw's shape can pull things together or push...