Passage: Soon after its introduction, the bottle became extremely popular with the soft drink and brewing industries mainly in Europe, India and Australasia, though some alcohol drinkers disdained the use of the bottle. One etymology of the term codswallop originates from beer sold in Codd-neck bottles, though this is generally dismissed as a folk etymology.The bottles were regularly produced for many decades, but gradually declined in usage. Since children smashed the bottles to retrieve the marbles, vintage bottles are relatively scarce and have become collector items; particularly in the UK. A cobalt coloured Codd-neck bottle can fetch hundreds of British pounds at auction. Bilas, a company in Portugal created a soda named after the bottle design (Pirulito). However, the marble inside did not seal the liquid. Instead it was a normal cap which could be removed without breaking the bottle.

Question: What is the main cause of vintage bottles becoming collector items?
children smashed the bottles