Toys: Playtex

How to Clean Playtex Sippy Cups?

Hi moms! My almost 16 month old son has been off the bottle since 12 months and is doing great with a sippy cup. We use the Playtex brand and they have been wonderful. However, we have noticed recently that no matter what we do to clean them, the milk and juice tends to really stick to the hard to clean areas. We do not have a dish washer. Do you have any advice for a safe, effective way to clean them? Thanks!