Task A
You're asked to put up a painting. The painting has a long thread attached to its upper corners.
You should hang up the painting with two nails in a way such that if one (any one) of the two nails is removed, the painting falls down.
Task B
The same task as task A, but with three nails. That is, you should hang up the painting so that it falls down if one (any one) of the three nails is removed.