http://news.yahoo.com/news?tmpl=story&u=/ap/20040707/ap_on_en_ot/translucent_concrete_1

WASHINGTON - It used to be only Superman who could see through concrete walls, but an exhibit at the National Building Museum shows mere mortals can do it too.

Called "Liquid Stone," the show features variations of translucent concrete, a newfangled version of the old construction standby that offers a combination of aesthetics and practicality.

One display is a wall of translucent concrete blocks. When someone stands in front of it and light is shone from behind, the person's shadow can be seen clearly on the other side.

"I think it's beautiful in itself, so it might be attractive in a restaurant or a hotel," said G. Martin Moeller Jr., the museum's senior vice president. "But it might also be used in an indoor fire escape where you wanted light to come through in case of a power failure. It could become a lifesaver."

The picture on the site is a must-see. Found on BoingBoing.