Lithops native to south africa are related to Livingstone daisies. Only ever producing two leaves per plant at a time, they are fascinating to have on any windowsill as they love full bright sunlight but wont grow too big! Surprisingly if kept dry in winter they can easily achieve -8c.
 
Keep watering to a minimum, in spring and summer water every 2-3 weeks and in autumn stop watering until spring. If the leaves start to shrivel give them a light water just to re-inflate them but otherwise wait until spring. Pot into a good quality soil based gritty compost.