Plastic bottle tops

And had they actually ever been collecting them?

I don't know if it's a scam or an urban myth that there's money in plastic bottle tops, but if you are collecting something for a charity it's always worth checking first or periodically that they are definitely (still) interested.

The mystery continues. No one can explain why it's always plastic milk bottle tops and not a more valuable recyclable such as steel or aluminium drinks cans?

Could this be a dated phenomenon? And no-one has the heart to tell the bright-eyeds when they roll in their balls? In the olden days (not too many decades into the previous century) foil caps were pretty much all foil (rather than the plasticky hybrids that have replaced them.) Nearly everyone drank milk and it was a universal way that all children in a class or group could contribute to the collection. And in those days fizzy-pop cans were a luxury - they were not bought regularly. Fizzy pop was only bought by families in 750ml or larger glass bottles. So not everyone could collect empty cans. (Course, doesn't explain lack of food tins - didn't have health and safety worrying about children ripping their hands apart on edges - so that blows up this theory.)