  We reconsider some fundamental problems of the thin shell model. First, we
point out that the "cut and paste" construction does not guarantee a
well-defined manifold because there is no overlap of coordinates across the
shell. When one requires that the spacetime metric across the thin shell is
continuous, it also provides a way to specify the tangent space and the
manifold. Other authors have shown that this specification leads to the
conservation laws when shells collide. On the other hand, the well-known areal
radius $r$ seems to be a perfect coordinate covering all regions of a
spherically symmetric spacetime. However, we show by simple but rigorous
arguments that $r$ fails to be a coordinate covering a neighborhood of the thin
shell if the metric across the shell is continuous. When two spherical shells
collide and merge into one, we show that it is possible that $r$ remains to be
a good coordinate and the conservation laws hold. To make this happen,
different spacetime regions divided by the shells must be glued in a specific
way such that some constraints are satisfied. We compare our new construction
with the old one by solving constraints numerically.
