The other day I mentioned to Hero that the baby had hiccups. She questioned, "How can it have hiccups when there is no air in there?" I told her that the baby swallows the fluid and it probably gets hiccups when it gulps too much. I looked online later and found out that while we know what hiccups are (involuntary contractions of your diaphragm), we really don't know for sure what causes them. Here was one possible solution: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9685564&dopt=Abstract
Try explaining that to a kindergartener.