Teacher: So you see, we now have only three weeks before the biology exams.
Student: That means I will only have three more driving lessons before my driving test.
Teacher: Uhh.
Student: Sorry sir, just a bain-fart.
When you mean to do something really obvious but then you forget what you were doing and end up doing something something completely different. i.e, when you have a wrapper in one hand and food in the other, and throw away the food and try to eat the wrapper.
Brain Fart:
Person 1: Man, I had such a brain fart today.
Person 2: What did you do?
Person 1: I was holding food and a wrapper and threw away the food and tried to eat the wrapper.
When you are attempting to remember something very obvious, someting that you know you should know. This feeling often leads to head banging and hair pulling.
bobby raised his hand to answer the teacher's question only to find that he had had a brain fart, and he was unable to answer 2+2.