And because 5e doesn't care about body parts, seeing even a fingernail means that you can hit them for full magic missile damage.
Lets put aside for a second that tooltip specificly say "a CREATURE that you can see" not "any small part of the creature you can see" ...
LETS SAY ... for a second ... that you are right, and you should be able to hit enemy by his fingernail ... you still have to aim for that fingernail, bcs you simply dont see anything else.
If your Darts are flying on curve, instead of flying straight ... you simply need to aim so that curve ends on that fingernail, otherwise you can never hit ... logicaly.
Therefore if you aim so your dart ends on any obstacle, you hit the obstacle. Easy as that.
