I'm just guessing that everyone out there knows what plastic is. The word plastic means pliable. Plastics are materials that can be shaped and molded easily. Plastics become easier to mold and shape when they're hot, and they melt when they get hot enough, so we call them thermoplastics. This name can help you tell them apart from crosslinked materials that don't melt. These materials can seem a lot like plastics, but they're really thermosets.
So we make things out of plastic by getting it really hot until it melts and then pouring it, stretching it, injecting it or blowing it into molded shapes, like drinking glasses and toys. It takes a lot of energy to deform plastics into shapes, but they stay the same shape. Until it gets hot enough, plastic cannot easily be deformed.
Hard Plastic and Soft Plastic
Of course, we've all seen plastics that are hard, and some that are soft. The plastic keys on your keyboard are hard, while the plastic around the cables of the same computer is soft. This is because each plastic has a certain temperature above which it is soft and pliable, and below which it is hard and brittle. This is called the glass transition temperature, or Tg, because the state of being hard and brittle can also be called glassy.
The Tg is different for each plastic. At room temperature, some plastics are below their Tg, and so they're hard like polystyrene. Other plastics are above their Tg at room temperature, and so these plastics are soft like polyethylene.
For example, suppose we have a piece of plastic has a Tg of, say, the same temperature as ice. In the freezer, that plastic will be hard and brittle, like glass. If you try to bend it, it'll probably shatter like glass would. At warmer temperatures, it'll be more pliable, or bendy. If a different plastic has a Tg the same as your body temperature, it will be hard and glassy at room temperature, and will get more pliable if you hold it in your hand long enough.
Now, don't confuse the Tg, glass transition temperature, with melting point. Although they SEEM similar, the melting point is different - just above the melting point, a substance is a liquid. Just above and below the Tg, a substance is still a solid, but a different type of solid.
Sometimes additives are added to a plastic to make it softer and more pliable. These additives are called plasticizers.
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