only read the question, not all the answers, so apologies if someone has already answered. I had the same thing - because the caps didn't come with the steriliser and bottles we bought (avent). We bought them separately in supermarket (pretty much anyone that sells the bottles). they fit into the rim (the bit the teat goes through). no harm putting the teat and lid on and shaking it - but I suppose some may spill into the lid? not really sure why they don't sell the caps with the bottles unless its because you don't really need them??