Hanging a picture can actually show a lot of physics dealing with force and friction.
First, you have to hit the nail with a hammer into the wall. Each time you hit the nail, you have to overcome the force of friction between the wall and the nail to get it to go in further. The hammer rebounds back after hitting the nail and you feel the force in your hand.
Then, depending on how heavy the picture you hang is, the wall has to exert a force equal to m*g on the nail to overcome the combined weight of the picture and the nail. If it exerted a force less than mg, the nail and picture would slide down the wall, leaving you with a large crack. Any larger than mg and it would accelerate upwards.
If you have two nails, the picture can be better supported because the wall can split the same force between two, so for a heavier picture use more nails.