It has to be a completely ideal box. That's a very extreme condition, so it messes very badly with people's intuition. It can not be a metal box with a cat in it, for example.
"It's a thought experiment" means "we can specify a totally ideal box." It doesn't mean "a finite metal box on a table will actually behave this way."
Schrödinger's Cat was intended to be easily imagined, but also obviously ridiculous.
I think that ends up confusing people a lot of people who can relate to the scenario, but don't understand that Schrödinger was accentuating the absurdity of the concept of superposition.
I really think that if the Cat was not a Cat, and if the binary was something other than the accessible-but-complicated concept of alive/dead, the thought experiment would be more useful to those of us who were not in the original messageboard threads between Schrödinger and Einstein.
This. Schrödinger's opinion was that the superposition collapses at the very beginning when the geiger counter goes 'click', after which everything behaves normally.
"It's a thought experiment" means "we can specify a totally ideal box." It doesn't mean "a finite metal box on a table will actually behave this way."