Always invisible, yet never out of sight. What are they?

Why this works

This clever riddle plays with the concept of visibility and presence in language. The letters "I" and "S" are always part of the written word but are often overlooked, making them "invisible" in the sense that we don't typically focus on individual letters when reading. However, they are "never out of sight" because they are essential components of countless words we encounter every day, particularly in the words "invisible" and "sight" themselves, which contain both letters. So, while we might not see them as distinct entities, they are always there, shaping our understanding of language!