This aphorism is about not exceeding your limits. If your head is made out of wax, then it will melt if you go out in the sun. Similarly, if you have a big exam, you shouldn't go party the night before, or any similar comparison that you can think of.
Alot of things! A dual can have many choices of what to be!
It's hot outside today.
It's is the contrasted form of it is and its is the possessive form *as it is without an apostrophe*.
For example:
The tree has lost all its leaves.
It's my birthday today.
The loss of his eye-sight
