Molting Photos & Video
A lobster grows by a process called “molting.” During this process a lobster will take in excess water which pushes against the shell from the inside, causing it to split. The lobster then extracts itself as it sheds its outer shell and emerges with a new, soft shell as its exoskeleton. This extraction is known as ecdysis. 
A thorough description of the molting process is available here on the Lobster Conservancy website.
Capt. Joe (Joe Ciaramitaro) has allowed us to link to a fabulous video and series of photos of a lobster molting, available on his blog Good Morning Gloucester. Thanks, Captain Joe.