Logic!

Today, I did something really stupid... In one of questions in the test, we were supposed to draw the structure of triglyceride. At first I did draw 3 oxygen atoms between the carbon atoms of glycerol and the carbon atoms of the fatty acids. However, I thought I don't see the oxygen atoms in the notes, so I merrily earsed them away, though I asked myself "Where the extra oxygen go to?" In the end, I flipped my notes, and....I saw that there should be oxygen there.

Moral of story: Use logic, rather than relying on your memory. The latter can fail you sometimes.