Let me preface this be saying I never write reviews, however when I saw how many people were saying mold was growing in their cups I was horrified so I made a video of me washing ours. My son was pretty much exclusively breastfed and wanted nothing to do with bottles so I got these cups for him to use so I could start introducing formula at 6 months. He figured out how to get the formula out right away and although we’ve had to help him hold the cup once he hit 9 months he has figured out how to hold the cup on his own and tip it back enough to drink. Cleaning is really easy you just need to make sure you disassemble all of the parts. The o ring is easy to take out, just look for the gap in the treads and you can take pull it down-without using any tools or nails! We sometimes don’t wash all the parts for a day or two when we are busy but we will put them in hot soapy water for a bit if we do that to get rid of dried formula. I would sugguest emptying the cup of any liquid the same day just so it doesn’t stink later. We also try to shake out some of the excess water before setting it out to dry.