It is strongly recommended that you try
this one first.
Can you hang a painting from three pegs such that the removal of any single peg causes the painting to fall?
Put a peg in the wall, place the painting on top of it such that the
peg is at the midpoint of the bottom side of the painting. Now put to
pegs at either side of the painting. If the pegs are round, and the
bottom one is sitting approximately in the middle, this should be a
valid solution (if we assume the pegs are large enough to support the
painting).
+---------+
o| |o
| |
| |
+---------+
o
solution without string
