Later they evolved into more elaborate designs made of pottery , taking the shape of faces where the smoke and steam from cooking would pour out through the eyes and mouth , or into constructions that were more like modern louvers , with slats that could be opened or closed by pulling on a string .
They took the shape of faces where the smoke and steam from cooking would pour out through the eyes and mouth . They also became more like modern louvers , with slats that could be opened or closed by pulling on a string .