Science feed
C
Diamond and graphite are different structural forms of carbon, but an ordinary pencil core is usually graphite mixed with clay or other binders. Their contrasting properties come mainly from how the carbon atoms bond, not from being different chemical elements.
0.01°C
Water can coexist as ice, liquid and vapor near its triple point, about 0.01 °C and 611 pascals. In a vacuum demonstration, evaporative cooling can make liquid water freeze while it is still boiling.
Low contrast
The Great Wall is generally not discernible with the unaided eye from low Earth orbit, and it is not visible from the Moon. Saying it can never be seen from any altitude called space is too absolute: viewing conditions, equipment and identification all matter.