There's a couple different categories here. There's the people that try really hard, but can't seem to grasp it, and then there are the people that sit in front of you in your sophomore science class that do nothing but make farting noises with their arms and talk about how "juicy" it sounded. Why yes, that did come from personal experience.
The first kind isn't a problem. I'm a pretty patient person when it comes to people actually trying. I don't care how "good" they are, when they're trying, it's good enough for me.
The latter, however, really gets under my skin. I don't really know how to deal with it, so I ignore it. When your teacher pairs you up with one of those kids in the same science class and I can't ignore it any longer, then we have a problem. Especially when your partner tried to convince you that they somehow managed to get all the way up to sophomore year in high school without being able to read and tries to make you read the instructions to him because he "lost his" within the first five minutes. From my rant you can probably tell how annoyed I get at people like this.