Mary Hawkins, age 8, of Jackson, Miss

What causes an echo?

You can often make an echo when you stand in front of a steep cliff  Walk back from the bottom of the cliff about 400 yards, which is about 400 giant strides  Than shout a loud hello with all your might, In about two seconds, a faint copy of your voice echoes back to your ears  No, there is not a small copycat hidden high in the cliff, The magic is done by the air, It carries the sound of your voice to the hard wall of the cliff and bounces it back for you to hear 

The air around us seems to be made of nothing at all, But actually it is made of tiny, tiny molecules much too small for our eyes to see  There are millions of these molecules in dust a thimbleful of air, with plenty of space between them and plenty of room for them to rush around at breakneck speeds 

You can think of the air molecules as countless small balls rushing around through empty space, colliding and crashing into each other like a mob of reckless drivers  The sounds we hear are made by these Jostling and dumping molecules of air, The sound of your loud Hello starts in your voice box which is deep in your throat  There you have two small bands of skin called the vocal cords  Sometimes the vocal cords lie loose and relaxed  But when you talk, they are pulled tight and they tremble with tiny vibrations 

When you shout a loud Hello, you use a big, strong breath of air to make your vocal cords shake and vibrate, The vocal cords jostle the tiny air molecules next to them  ‑ and the sound starts  The first molecules jostle the molecules behind them and the jostling is handed on and on through the air  In less than a second, this jostling carries the sound of your voice about x;00 yards,

All this Jostling needs a good push of energy to start it off  This push is the breath of air coming through your voice box  But as the jostling travels on and on, the energy is gradually used up and lost  The sound gets fainter as it travels father and farther from your voice box  A small push of energy from a small breath of air makes a soft sound which soon fades away, T o make a loud Hello, strong enough to travel to a cliff and echo back, you need a big push of energy from a big breath of air 

This makes the air molecules jostle each other all the way to the cliff  They cannot go through the solid rock and they still have some energy left, which they must use, The molecules now start jostling each other in the opposite direction  The sound bounces off the wall of the cliff and comas echoing back to your ears  But the Jostling energy is almost spent and the echo is just a faint copy of your loud Hello 

If you shout Hello, Hello, chances are you will hear an echo of only the second Hello  This is because the first Hello echoed back while you ware shouting the second one and you dust did not hear it.