I've seen a lot of crazy and inconvenient space-saving solutions in residential buildings in New York, Hong Kong, Tokyo. So I'm amazed I've never seen anything like this two-way staircase until now:

The model above was realized as a full-scale art installation called DIAGINT, created by NYC-based artist Ian Stell on the banks of the Spree River in Berlin.

"Building codes dictate the strictest of formulas for the design and construction of stairways," he writes. "Yet despite the fact that stairs are the most reliable means of egress, they are treacherous: As with their cog cousins, they bare their teeth in perpetuity."

I can guarantee you that if NYC landlords could bribe the right inspectors and have these installed, inconveniencing tenants while gaining extra space to profit from, these'd be in buildings all over the city.