Protests against police brutality are currently taking place across the country, and while technology has been helpful in organizing them, it can also be used against those in attendance. Facial recognition can be used to identify (or misidentify, as often happens) protesters and phones can be surveilled.
If you’re demonstrating, it’s a smart idea to take steps to lock down your phone ahead of time or even just leave it at home. If the latter isn’t possible, put it in a Faraday cage, an enclosure that shields against the entry or escape of electromagnetic fields.
A Faraday cage is useful for many circumstances. Perhaps you have a car with remote keyless entry, and want to prevent it from getting broken into by teens with a power amplifier. Or you don’t want the NSA listening in on your conversations. Or you’re a doomsday prepper just waiting for a gigantic, Earth-enveloping electromagnetic pulse to rise up in the sky and wipe out all electronic communication.