So called because of the sound they make, in this case I've put the flop before the flip. Well why not ?!
Flip-flops are one of the oldest forms of footwear, and first found in Egypt in 4,000bc! They entered the modern world when American soldiers brought them back from Japan after WW2. They then entered hippy culture and became a must have.
In different countries they're called different things. Here's a small selection....
This collection of photos have been taken around the world. These are the most successful, cheap and practical footwear ever made.