The father bought gifts for each of his three daughters, whose names were Black, White, and Gray. Thinking it would be amusing, he bought two black scarves for Black, two white scarves for White, and a white scarf and a black scarf for Gray. He had the store clerk wrap the three presents and label them with the names of his daughters.
But, there was a momentary distraction. The scarves did not get mixed up, but all three labels were wrong. By lifting a corner of the wrapping on any one of the gifts, it is possible to see the colour of one of the two scarves in that package.
How many corners does the father have to lift to determine what the correct labels should be?