If somebody is said to be once bitten twice shy, it means that someone who has been hurt or who has had something go wrong will be far more careful the next time. It’s something that you say when you have had an unpleasant experience and are going to be more careful to avoid a similar experience in the future.
Transitions lenses contain special patented photochromic dyes that cause the lens to activate, or darken, when exposed to ultraviolet rays from sunlight. When the UV light diminishes, the lenses fade back. As light conditions change, the level of tint adjusts, offering the right tint at the right time.