strange bang(history bash expand)

Question

In bash:

$ a=1
$ echo $a
1
$ echo "$a"
1

But:

$ echo !
!
$ echo "!"
bash: !: event not found

Why in "echo !", bash don't expand.

Answer 1

In "echo !", after bang followed the carriage return.

man bash

! Start a history substitution, except when followed by a blank, newline, carriage return, = or ( (when the extglob shell option is enabled using the shopt builtin).