When you 3D-print an object, that's usually all she wrote -- the polymers "die" and prevent you from either adding to your creation or altering its properties. However, things might not be set in stone (or rather, plastic) for not much longer. MIT researchers have developed a method that lets you modify polymers and mess with your original creation. The key is to create accordion-like 'living' polymers that stretch when you expose them to blue LEDs, giving you a chance to alter their properties. You could fuse objects together, soften them, or even alter their water resistance.