Boundary of a manifold

Think of a map with different countries on it. Each country is like a different neighborhood or region in a city. Now, imagine drawing a line around the edge of each country. This line is like the border or boundary of the country.

In the same way, a manifold (a fancy word for a space that looks like a flat surface) also has a boundary. The boundary is the edge of the manifold. But instead of being a straight line like the border of a country, it can be curved or twisted depending on the shape of the manifold.

For example, imagine a balloon. The surface of the balloon is a manifold, and the boundary is the edge where the rubber stops and the air inside begins. The boundary is curved since the surface of the balloon is curved.

So, the boundary of a manifold is the edge or surface that separates the inside from the outside. It can be straight, curved, or twisted depending on the shape of the manifold.