Why does a tea bag float in cold water but sink/immerse in hot water ?

A few reasons spring to mind but someone?s doubtless done a PhD on the subject.

1) The heat expands the air, expelling it.
2) The fluidity of hot water increases with temperature so it is able
to penetrate and replace the air more effectively.
3) The density of water falls with temperature, probably more than tea.
4) The hot water kills the teabag and it stops swimming.

Best