Refrigerate the phones

Snowden was asking visitors to hide their cellphones in the fridge of his home in Hong Kong, to block any eavesdropping. 

Why a refrigerator? The answer does not, as some might assume, have anything to do with temperature. In fact, it does not matter particularly if the refrigerator was plugged in. It is the materials that make up refrigerator walls that could potentially turn them into anti-eavesdropping devices.

Good to know, for future reference.