I will give you an example of why even very simple mathematics can be very strange. Lets say I borrowed $100 from a friend, went into a shop and lost them. Then I met a girlfriend and borrowed $50 from her. I bought two chocolate boxes worth $10 each. Now I am left with $30. I gave these $30 to my friend, now I owe him $70, and I owe my girlfriend $50, thats $120 in total,  plus I have 2 chocolate boxes, thats $140. Now my question is the hell is an extra $10?

Now how interesting is this, such a simple exercise and yet we manage to loose $10 as a result.