Most of the eyeball is filled with a clear, jelly-like substance, similar to a raw egg white, called the vitreous body. In children and young adults, this jelly is more solid. With ageing, areas within the jelly begin to liquefy and gradually become fluid. As this natural degeneration proceeds, a stage is reached when the jelly suddenly collapses inward on itself toward the centre of the eye (an acute vitreous detachment). Small strands of condensed jelly become suspended in the mobile fluid and move about in the eye casting a shadow on the retina in the back of the eye, creating the small floating shapes. The jelly has stronger attachments to the retina on the inner wall of the eye in some areas. With eye movement, the jelly wobbles, and may tug on the retina where it is still attached, generating lightning-like flashes. Occasionally, as the jelly collapses away from the wall of the eye, it creates a small tear in the retina due to the pulling. This can then lead to a lifting off of the retina (a retinal detachment). Sometimes, bleeding can occur from a torn blood vessel associated with a tear. While these processes most commonly result from normal ageing in the eye, degeneration of the jelly occurs far earlier in people who are highly short-sighted or can be precipitated by a blow to the eye.