A company in New York has invented a paint that can switch between blocking cell phone signals and allowing them through.

Using nanotechnology, particles of copper are inserted into nanotubes,
which are ultra-tiny tubes that occur naturally in halloysite clay
mined in Utah. Combined with a radio-filtering device that collects
phone signals from outside a shielded space, certain transmissions can
proceed while others are blocked, the Chicago Tribune reported.

Pretty cool. Sure, there are some safety issues to consider, but I think everyone can agree that blocking cell phones at movies and concerts would be a good thing.