HIS BODY 93 
Birds have very large gullets. In many cases 
they lead into a place called the crop, where food 
is kept before it goes into the stomach. Some- 
times the food is made soft in the crop, and then 
fed to the young ones, as I told you. 
Birds have no teeth, yet they eat hard seeds, 
like acorns and grains of corn. To break these 
up, and get them ready for the stomach, they 
have a gizzard, which is a sort of grinding-mill. 
And to help in the work of grinding they swal- 
low small stones. 
One of the wonderful things about birds is the 
height at which they can live, and not only live, 
but fly. A man cannot go higher than twenty- 
two or twenty-three hundred feet, while moving 
about or exercising, because the air is so rare he 
cannot breathe. The highest a man was ever 
known to go and live, it is said, was less than 
thirty thousand feet, and that was in a balloon, 
where he did not move. 
But birds go a good deal higher than this, 
and can fly — which is violent exercise — at 
that height. It is thought by some that the rare- 
fied air may be the cause of the great speed with 
which birds fly in that region. But there is still 
much to be found out about this. 
Besides the marvels of flight, birds have other 
powers almost as strange. Many of them can fly 
