Why is the number 21 so significant?
Satoshi's birthday, 4/5/75, has digits that sum to 21 (5 + 4 + 7 + 5). The date of block #1, 1/9/2009, also sums to 21 (9 + 1 + 2 + 9). The halving happens every 210,000 blocks. Until the 21 million #Bitcoin are exhausted, in 2140.