American scientists have developed the first material that repairs itself. The material is a form of plastic that has been engineered to fill breaks in its surface. Plastics are used today in everything. Scientists wanted to find a way to make objects made of plastic last longer. The researchers hope their new discovery can be used to make objects that are difficult or impossible to replace. The surface of plastic objects breaks over time. Very small breaks develop every time a plastic object is used. The researchers wanted to find out how to stop plastic from developing small breaks that weaken and destroy it. They found the answer in the chemical structure of plastic itself. The research team found a way to make plastic that contains very small balloons filled with monomer  liquid.  When the new plastic cracks, the monomer liquid is let out and flows into the crack. Thus the break is repaired. The repaired plastic has seventy-five percent of the strength of undamaged plastic. The new self-repairing plastic is not yet ready for production. But it has several possible uses. One could be in space vehicles where parts cannot be repaired or replaced. Another possible use might be in bridges. And it could be used in people, to replace bone joints that have become broken or damaged. We know from the passage that the repair is done as  _  .
the cracked plastic parts are changed the plastic cracks the liquid is put into the plastic cracks the plastic is made