IDIOM ►

like a sitting duck

Meaning 1
as a duck sits on water
Sentence 1
That target practice was easy, like hitting a sitting duck.
Meaning 2
to be unaware of something about to happen
Sentence 2
The ememy's troops were like sitting ducks. They did not suspect an attack.

Derivation

A sitting duck is easier to shoot than one that is flying. A stationary target is easier to hit than a moving one.