Subject: Why are some ice cubes 'harder' than others?

Otentimes, I get ice cubes that are very "hard" to "chew!" Othertimes 
they can be chewed quite easily. Why?

A similar observation is that some ice cubes melt right readily in 
your drink and othertimes those from another source will last for a 
relatively long time. In both of these "melting" cases the drinks are 
precooled and the the cups are essentially the same type of thin "wax" 
impregnated paper as used by fast food restaurants.

Re: Why are some ice cubes 'harder' than others?