Because when you gaze at an attractive woman, look deeply into her beautiful eyes, try to say something inteligent and watch her smile........ your heart is not getting cold.
My SWAG -- Scientific Wild-A$$ed Guess -- is that in the early days of pipelining, the root was run fairly 'cold' and fast. This closes up the open gap in the root. Then comes a hotter, slower pass to tie everything a little better: the Hot Pass.
If you watch any root when the 2nd [hot] pass is being welded, it does indeed get very hot, and portions of the root may remelt.
