Kobold beliefs represent the survival of pagan customs into the Christian and modern eras and offer hints of how pagan Europeans worshipped in the privacy of their homes. Religion historian Otto Schrader has suggested that kobold beliefs derive from the pagan tradition of worshipping household deities thought to reside in the hearth fire. Alternatively, Nancy Arrowsmith and George Moorse have said that the earliest kobolds were thought to be tree spirits. According to 13th-century German poet Conrad of Würzburg, medieval Germans carved kobolds from boxwood and wax and put them "up in the room for fun". Mandrake root was another material used. People believed that the wild kobold remained in the material used to carve the figure. These kobold effigies were high and had colourful clothing and large mouths. One example, known as the "monoloke", was made from white wax and wore a blue shirt and black velvet vest. The 17th century expression "to laugh like a kobold" may refer to these dolls with their mouths wide open, and it may mean "to laugh loud and heartily". These kobold effigies were stored in glass and wooden containers. German mythologist Jacob Grimm has traced the custom to Roman times and has argued that religious authorities tolerated it even after the Germans had been Christianised.