From the UTF-16's wikipedia entry, the second sentence states it's a variable length encoding.
But where is the separator between a 16-bit character and 32-bit encoding? I know a lot of characters can be stored in the 16-bit character, so people can optimize UTF-6 that way, but I'm still curious because there could come a time when we could surpass the 16-bit mark.
And no worries, I know UTF-8 is standard. I'm just curious.