(1) We usually do not think of animals talking, but animals do make sounds. (2) Some fish can make sounds by grinding
together the bony parts of their throats. Crickets rub their
rough wings together. The chirping of a bird is made when it
forces air through its throat. (3) Birds can also make drumming
sounds with their feathers and feet. (4) The rattle of a
rattlesnake and the slap of a beaver's tail are also examples of
the ways animals communicate.
Which number is the topic sentence