A thread on today's SXS preprint: https://t.co/ry9i6UtSSG
In order to write very large, or very small numbers efficiently, scientists use a system of writing in powers of 10. For example, the number 108 represents 10 multiplied by itself 8 times, which is a 1 with 8 zeroes after it: 100,000,000. Thus, rather than writing 300,000,000, a scientist will write 3 × 108. Similarly, a number like 10-21 represents 0.1 multiplied by itself 21 times, which is a 1 with 21 zeroes in front of it (including the one before the decimal point): 0.000000000000000000001.