It's Like taking candy from a baby

[Kirstyhev]

why would this mean that something is easy?

[Quizmonster]

Well, how difficult would it be to take a candy-bar from the hand of a child? Trying to take one from a heavyweight boxer's hand might be a bit of a problem, however!
There are other similar sayings such as 'It's like shooting fish in a barrel.' Mind you, given that the refraction of light would show the fish to be in the 'wrong' position - ie you might easily miss - that might actually be relatively hard!

[deso]

I tried to take some candy from a baby once and it's mum twa**ted me. Babies usually have back-up, they're rarely alone, so it's not as easy as you might think.
I once shot a barrel of fish with a howitzer. It was easy.