“ Believable magic, like ordinary physicis, operates according to some invariable laws that always result in some kind of cost or 'bounce back'. The grater the magic, the more it should cost the character physically, emotionally, and spiritually.”
“ The Chinese, when attacking a castle or fort, flew kites over the city wall and used the length of string it took to get it there as a measurement to know how far they had to dig a tunnel to get under the wall.”
“ A continent is home to large, leathery flying beasts with a wingspan of over 70ft. Though shy, herbivorous and easily scared, it is possible that an enterprising person could domesticate them. They would be useless in war due to their timidity, but excellent if used for commercial air travel.”