User blog comment:Cherryberry1456/New Discoveries!/@comment-5902187-20131031001538/@comment-5902187-20131031004836

Possibly to hold bubble soap, and you could open that compartment up when it's bathtime. I doubt they'd make it permanently open, where you can spill bubble soap on your hands whenever you play with them. Storing the bubble soap in a compartment in their head would be a good idea (if it's in a way that wouldn't mold, if bubble soap even molds) so that you wouldn't have to keep re-filling it all the time.