A few weeks ago I pulled a little prank on him which caused a bit of temporary confusion and at the height this confusion I *might* have implied he was losing his mind just a bit. After he caught on to the prank and realised I had just been pulling his leg... he started to sing a song he made up. In a sing song voice for numerous blocks he sang his little ditty. The lyrics went something like this, "I'm not insane, I'm not insane, la la - de la da, I'm not insane." The song was a familiar tune... perhaps Homer Simpson singing "I am so Smart."
In a very serious voice I asked "How can you be so sure you aren't insane?" hoping to just put a wee bit of doubt in his mind.
With total confidence (and completely deadpan) he replies "Because I have a song."
Who can argue with that logic?