Today I'm hanging the unseen River Scrolls using these small bu powerful neodymium magnets. Here's a wiki-blurb:

The greater force exerted by rare earth magnets creates hazards that are not seen with other types of magnet. Neodymium magnets larger than a few centimeters are strong enough to cause injuries to body parts . . .  If possible, one should bear the pain and do whatever one can to keep the distance from closing up; only then should one pull free.

They are NOT kidding.