I discovered a great way to clean the tops of sippy cups today. My sons cups get build up on the inside where the spout is. I was trying to figure out what to use to get it clean. I was ready to throw them away. Which I should probably do anyways. I got out our barbecue brush which we don't use, the hubby doesn't like BBQ sauce for some reason. It has long enough bristles to get way down in the cracks. It worked wonders. You could try a toothbrush but I'm not sure if it will be long enough. We use the Gerber cups.