Imagine you are walking in the street and someone comes to you asking directions.
Ok !! You start telling him how to go there. Then out of no where, suddenly, two workmen walk between you two, carrying a huge door !!
Interestingly, the guy you have been giving direction quickly changes with one of the door carrying workmen... When the door is gone ... you are left giving direction to a taller, different man !!!!!
Do you think you will obviously notice that ????
15 people have been tested with this situation, 8 of them did not notice that the guy he had been giving direction has changed :D
Pay attention !!
Attention is something not well known, but surely it's a limited resource :)