Suppose you travel to a third world country, for example Romania. You arrive in the capital city of Bucharest, late in the night, and want to exchange a $100 bill to the country’s currencies, which are called "lei". All exchange offices are closed. A local citizen approaches and proposes you to exchange your bill. He is a thief.
You give him the $100 bill, he gives you the equivalent in the country’s currency, i.e. 25,000 lei. But the laws of the country do not allow exchange on the street, and both of you know it.
The thief cries "police!", and gives you the dollars back with one hand, while with the other hand takes back his lei, and runs out vanishing behind a building.
The thief has cheated you.
Taken by surprise, you don’t realize what had happened, and looking in your hand expecting to see back a $100 bill, actually you see a $1 bill… in your mind, in the very first seconds, it appears the illusion that the $100 bill changed, under your eyes, into a $1 bill!