2 Questions I have the answer need explanation how to get it

1.) The revenue recognition principle states that revenues should be recognized _______________________ the answer is: In the period they are earned. why can you explain

2. Cliff Corporation signed a one-year $20,000 note payable at an interest rate of 6% on December 1, 2020. How much interest expense should be accrued on December 31, 2020 the answer is: $100. can you explain why