MASTER KEYS
Credit: NY Times Crossword
Frances Hansen  (Idea and #4, 18, 20)

The answers to the clues below will have something in common.

1. Completely out of money
2. Very quickly
3. Mentally alert
4. Severe comeuppance
5. Don't confuse them with the facts
6. Common simile for squashed
7. Beau Brummel type
8. Driving danger
9. Type of women's shoe
10. Utensils
11. Type of haircut
12. Speaking hurtful words
13. Bed-ridden
14. Type of cheese
15. Work vehicle
16. Podiatrist's concern
17. Car woe
18. Marksman
19. London apartment
20. What it's like to have a thankless child

Answers:

1. Flat broke
2. In nothing flat
3. Sharp as a tack
4. Sharp rebuke
5. Flat Earth Society
6. Flat as a pancake
7. Sharp dresser
8. Sharp turn
9. Flats
10. Flatware
11. Flat top
12. Sharp-tongued
13. Flat on your back
14. Sharp cheddar
15. Flatbed truck
16. Flat feet
17. Flat tire
18. Sharpshooter
19. Flat
20. Sharper than a serpent's tooth