The Two Things

For every subject, there are really only two things you really need to know. Everything else is the application of those two things, or just not important.
So here is The Two Things

A few favorites :

The Two Things about Accounting:
1. The trial balance must balance.
2. There's a lot of "grey area."
...
The Two Things about Software Engineering:
1. There is no such thing as bug-free software.
2. Adding manpower to a late project makes it later.
...
The Two Things about Teaching History:
1. A good story is all they'll remember, not the half hour of analysis on either side of it.
2. They think it's about answers, but it's really about questions.
..
The Two Things about Engineering:
1) It's all about tradeoffs.
2) The tradeoffs are all about money, time, and quality.
..
The Two Things about Economics:
1. Incentives matter.
2. There's no such thing as a free lunch.
..
The Two Things about Blogging:
1. Everyone who runs one is a kook.
2. Everyone who reads one is a kook.