The sun is the main source of energy for people, and they use it in various ways. The energy in fossil fuels, such as oil and coal, comes from the sun indirectly, because the fuels come from plants that grew long ago. Some energy sources cost less than others and some cause less pollution than others. Signed and spoken language was one of the first inventions. Early forms of recording messages used markings on materials, such as wood or stone.
Communication involves coding and decoding information.
In any language, both the sender and the receiver have to know the same code, which means that secret codes can be used to keep communication private. People have invented devices, such as paper and ink, engraved plastic disks, and magnetic tapes, for recording information. Communication technologies make it possible to send and receive information more and more reliably, quickly, and cheaply over long distances. Students explore their ability to discriminate between sounds, by dropping objects into a drop chamber and identifying each object by the property of its sound. Students develop a code by assigning letters to a selection of the objects and send messages to one another by using their drop code.