Free boundary condition

A free boundary condition is like a box with no lid - it's a way to describe a situation where there are no walls or limits. For example, imagine a lake - there's no restriction on how much water it can hold or how deep it can be, so the lake can be any size or shape it wants. In physics and math, a free boundary condition means that we don't need to worry about how big or small something is, or how it is shaped. It's like that lake - the boundary is free to choose whatever shape it wants, and can even move over time.