Chalk produce, and thus widely used in the developing world, calcium-based chalk can be made where the crumbling particles are larger and thus produce less dust, and is marketed as "dustless chalk". Colored chalks, pastel chalks, and sidewalk chalk (shaped into larger sticks and often colored), used to draw on sidewalks, streets, and driveways, are primarily made of gypsum. Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching of quicklime with water. In southeast England, deneholes are a notable example of ancient chalk pits. Such bell pits may also mark the sites of ancient flint mines,
Einstein's Blackboard for the expansion that was too high by a factor of ten. These errors were corrected in a later review of relativistic cosmology written by Einstein in 1945. A blackboard used by Einstein in a public lecture at the University of Nottingham on 6 June 1930 was also preserved after the lecture and is now in the university's archives. Einstein's Blackboard Einstein's Blackboard is a blackboard which physicist Albert Einstein (1879–1955) used on 16 May 1931 during his lectures while visiting the University of Oxford in England. The blackboard is one of the most iconic objects in the collection of