Is there a point where a teacher can say "i have done all i can to teach XZY to you. You ability of understanding XYZ now rests in you and no longer in me" ? 

I feel like there is nothing worse for a teacher than when students are incapable of understanding a simple idea/concept or procedure that has been discussed/taught for at least one week. 

For example, think about the use of a microscope. For the last two weeks we have used a microscope every single day. I have spent an entire class period introducing the parts and the use of the microscope. We have looked at dozens of slides and discussed them in class. I expect that at this point each student can make their own slides and bring it into focus under the microscope. This is important because we have reached a point where i can no longer invest time in talking about operating the microscope because the course needs to move forward. Still this seemingly simple task has escaped more than one student. Since my job is to teach i would never utter the sentences in the first two lines of this post and I will keep explaining the principle each time it is needed. I guess this where testing comes in to play. test=my revenge