Paper cup phones are a classic kids activity.
You will need
2 paper cups
inelastic string, thread, or fishing line
a pencil or sewing needle
Punch a hole into the bottom of each paper cup using the pencil or needle, thread your inelstic string through the hole in each cup (one cup on each end of the string) and secure to a paperclip inside the cup. This keeps the string from falling out of the cup. To use draw the string taut. One person speaks into the cup the other person put their ear to the cup and listens. Instant fun!
Do you have older kids who want to understand the science behind this?
Sound travels as a wave. Though technically not the same kind of waves, you can illustrate the idea of waves travelling through a medium with water ripples or by taking a jump rope and having one person hold still at the end while you make a wave travel from one end to the other by jiggling the handle. ***Technically, a sound wave is a compression wave (water ripples are circular waves, and jump rope waves are longitudinal waves). A compression wave can be illustrated using a slinky stretched out across the floor, shove one end back and forth and watch the compression move from one end to the other.***
A sound wave has to travel through something because sound is a vibration and has to pass from particle to particle. This is why there is no sound in space (contrary to sci-fi shows) Sound can not travel in outer space because there is no air, there is nothing but empty space, so the sound doesn't go anywhere.
Sound can travel well enough through air, but it travels even better along a solid. This is why you can whisper into a cup phone and the sound will be heard at a greater distance away than possible without the phones. When you speak into the cup, the bottom of the cup vibrates with the sound, these vibrations (or sound waves if you like) are then passed on to the string which vibrates (if held taut) all the way to the other cup. The string passes the vibration off to the bottom of the cup which then vibrates the same way the first cup did, thus transmitting the sound from one person to another.
Once again, in order for this to work the string must be pulled tight and really shouldn't be touching anything other than the cups.
See, science can be fun!
Want to turn it into an experiment? Try different kinds of thread or string, some elastic, some inelastic. Which transmit the sound the best? Hold the string limp and then tight, which works best? Try tying another cup phone onto the string, does it work?