Question

the teacher said the water boil is at 100 degree celsius( convert to direct speech)​

Answer 1

The teacher said, "The water boils at 100 degree Celsius."

《The tense does not change because it is a universal truth.》

《It must be 'boils' in the question, not 'boil is'.》

Answer 2

$\sf{\large {\underline {Direct\:Speech}}}$ :

The teacher said, " The water boils at 100 degree Celsius."

❇ This is the Universal Truth, So there will be no change in the tense of direct speech.