The 20th century Physicist Enrico Fermi made famous a class of problem that require order-of-magnitude estimates to solve. In many cases, the physical world can be understood quite accurately by order of magnitude estimates.
How much heat is required to warm all of the earth oceans by 0.1 C? (in Joules)
A. 5x10²⁰ J   B. 5x10²¹ J   C. 5x10²² J   D. 5x10²³ J   E. 5x10²⁴ J