The joke is, the second person heard 127 degrees, assumed that was Kelvin, and dismissed it as unimpressive. Then they realized it was 127 degrees Celsius, which is MUCH more impressive.

The punchline is the temperature was originally presumed to be Kelvin.

Though a _real_ stickler of a scientist knows that K is not given in degrees and would have lightly chastised if making that assumption.

I still enjoyed the joke though.