Kahneman, the brilliant Israeli cognitive scientist, says there are two ways we think:

System 1 and System 2.

System I is fast and System 2 is slow.

Here is a simple puzzle to illustrate how System 1 can fool us.

Do not try to solve it. Just listen to what your intuition tells you straight away!
 
A bat and a ball costs $1.10
The bat costs one dollar more than the ball.

How much does the ball cost?
 
(are you sure it's $10? System 2 is there to correct the intuitive answer System 1 "told" you is right!)