In the 1990s, the American toy manufacturer Tyco offered a series of spy kits for sale. One of these, the Electronic Secret Message Finder Set, contained a fake rock with electronic equipment inside that responded to a secret whistle. There was also a small compartment for hiding messages, much like geo-caching rocks today. In March 2013, one was offered for sale on eBay. The seller,Vintagemania10, may still have it! It's great to know that in the fake rock world there are classic rocks!