92 HOW HE IS MADE 
He can sing while he is working very hard to 
fly upward. If you will try to sing while run- 
ning up a hill, you will see how hard it is to do 
that. 
A bird’s head is joined to his neck at one place, 
something like a hinge. Other animals, like dogs 
and cats, have two hinges, or places of joining. 
That is why a bird is able to turn his head 
around so far that he can look down his own 
back. No other creature can do so. 
Because of this, he is able to dress every 
feather on his body, and to sleep with his head 
laid back on his shoulder. 
Nearly all birds have some of their bones hol- 
low, and air-sacs, or pockets, under the skin. 
These sacs they can fill with air and make them- 
selves light, so that those who live in the water 
cannot sink, but float like a cork. 
Men who study the way birds are made do 
not yet know all the uses of the hollow bones 
and air-sacs. That is one of the things left for 
you young folk to find out. 
Birds who get their food in marshes, or the 
edge of the water, have long legs for wading. 
They have also long necks, so they can pick up 
food from the ground. 
Birds who swim have webs between the toes, 
that turn their feet into paddles. 
