Yes, you read that right. Literally a rock in a box of hay, complete with breathing holes, this simple toy was invented by former advertising exec Gary Dahl in 1975, who marketed the rock as a pet that requires absolutely no caring for. Dahl came up with this unique idea when listening to his friends complain about having to look after their actual real-life pets. Dahl earned up to $15 million (£6m) during the mid-70s, despite the Pet Rocks only costing $3.95 (£1.60) each.
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