No, you're not looking at computer-generated optical illusions, but rather real clouds that were created by Dutch artist Berndnaut Smilde. Unfortunately, these clouds do not hang around for very long because they're created with fog machines. Continue reading for a video and more information.
The trick is to get the right level of humidity, temperature and lighting for it to look as real as those outside. Berndnaut shapes the cloud by using a fog machine. The surreal creation dissipates in a matter of moments, though, and the guy admits that a photograph is then the only "document" of its existence. Berndnaut "hangs" these momentary ephemeral installations in art galleries giving the viewers only a few seconds to see it.