[paul.fr]

I popped a chunk of polystyrene in to the freezer for 24 hours, when i took it out it was not frozen and actually felt warm!
Why would that be?

http://www.thenakedscientists.com/forum/index.php?topic=6730.msg171359#msg171359

[another_someone]

OK, the question took a little reinterpretation.  Polystyrene is a solid, so it is already frozen - so what you meant to ask is why did it not feel cold.

Again, I am assuming you are referring to expanded polystyrene, rather than simple polystyrene polymer.

Expanded polystyrene is a superb thermal insulating material.  The polystyrene foam itself will be cold, but because it is a good thermal insulator, it will not take a great deal of heat from your hand, so as your hand will not leak a lot of heat into the polystyrene, you will not feel any substantial loss of heat from your hand, but will simply feel the normal heat of blood flowing through your hand.

[lyner]

There are many, many substances which don't really fall into 'solid', 'liquid' or 'gas'. The states of many mixtures are quite indeterminate over large ranges of temperature. Over a range of temperatures, plumber's solder and motor car antifreeze are great examples of the 'porridge' state, half way between solid and liquid.

School Science insists on categorising things and you have to re-think a lot of this stuff if you want a better understanding of Science.
A real favourite with School kids is "Can you eat it or is it a POISON!!!?" no other alternatives seem to exist.