Recently, I heard about a psychology study that counted how often children laugh in a typical day (by the way, the study of laughter is called gelotology, apparently). The number is something around 300 times a day for a typical four-year-old. The striking thing is that the researchers found that an average forty-years-old adult laughs only about 4 times a day.
The hypothesis for explaining such a huge difference is that children acquire language skills at that age, and the English language (similar to most other languages, I suppose) has many expressions that are funny or absurd when taken literally (e.g. “to pick one’s brain”). The adults have heard these expressions so many times that they immediately perceive their implied meaning, without stopping to think, and to laugh at, the actual words.
Here is an example supporting this theory. After leaving in Milan for a few months, we are starting to pick up a few Italian words, and I find myself if not laughing out loud then at least chuckling when my daughter cries “Mamma mia!” when I drop a piece of pizza on my lap.
So my personal extension of the theory is that going on sabbatical makes us younger by forcing us to be child-like when faced with new languages, custom and everyday situations. We just need to come back home when things stop being funny.