David is standing behind Ashley, and Ashley is 
standing behind David. 

How is this possible?

They are standing back-to-back.
 

Bright as diamonds,
Loud as thunder,
Never still,
A thing of wonder.

What am I?

A Waterfall.

What has roots that nobody sees, and is taller than trees.
Up, up it goes, and yet it never grows.
What is it?

A mountain.

I run over fields and woods all day. Under the bed at night I sit not alone. 
My tongue hangs out, up and to the rear, awaiting to be filled in the morning. What am I? 

A shoe.

An iron horse with a flaxen tail.
The faster the horse runs,
the shorter his tail becomes. What is it?

A needle and thread.

I am something that nothing is, but yet I have a name.
I am sometimes tall and sometimes short.
I join your talks; I join your sport,
And I play in every game.
What am I? 

A Handshake.

Footsteps.