7 Really Complicated Things You Do Without Thinking

4. "1+1=2"

It's easy as 1+1=2, right? It's the first maths that most of us will have learnt as young children, and yet, this simple little equation gave some of the mathsiest mathematicians on the planet sleepless nights. To actually prove that 1+1=2, it took two mathematicians, Bertrand Russell and Alfred North Whitehead over 300 pages of noggin-squashingly dense maths in a work that is widely considered one of the most important works in mathematics since Aristotle. To give you some idea, this is just a tiny section of Russell and Whitehead's proof from their seminal work, Principia Mathematica. One of the reasons that the proof takes up such a lot of library space is that those stoic brainboxes had to first define what 1, 2, + and = mean, beyond any doubt before they could even begin throwing them together. Could be worse, they could have discovered after over 300 pages that 1+1 doesn't actually equal 2, and we would have all discovered that we've been splitting the bill wrong after all this time.