Paper consists mostly of cellulose, a natural polymer (i.e. plastic) built by stringing together thousands of individual sugar molecules into vast chains. Like the sugars from which it’s constructed, cellulose’s molecular pieces cling tightly to one another at room temperature and make it rather stiff and brittle. Moreover, cellulose’s chains are so entangled with one another that it couldn’t pull apart even if its molecular pieces didn’t cling so tightly. These effects are why it’s so hard to reshape cellulose and why wood or paper don’t melt; they burn or decompose instead. In contrast, chicle — the polymer in chewing gum — can be reshaped easily at room temperature.