Chalk It is formed in the sea by sub-microscopic plankton, which fall to the sea floor and are then consolidated and compressed during diagenesis into chalk rock. Most people first encounter the word "chalk" in school where it refers to blackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased. Blackboard chalk manufacture now may utilize mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to
What Are Little Girls Made Of? What Are Little Girls Made Of? "What Are Little Girls Made Of?" is the seventh episode of the of the American science fiction television series, "". Written by Robert Bloch and directed by James Goldstone, it first aired on October 20, 1966. The first episode of the series to be repeated on NBC, the title of the episode is taken from the fourth line of the 19th-century nursery rhyme, "What Are Little Boys Made Of?." In the episode, Nurse Chapel searches for her long lost fiancé, and uncovers his secret plan for galactic conquest. On stardate 2712.4, the Federation starship