Greatest easily replicable inventions of 20th century

By simple I mean things you can replicate in a reasonably short time just yourself, given you know ahead how to make them.

1. Diffie-Hellman asymmetric cipher
2. Game theory applied to prisoners dilemma, Nash equilibrium
3. Prospect theory. Demonstrate it on a questionnaire
4. Penicilin. Use bread
5. Nylon. Use it on Velcro - demonstrate it on shoe for kids. Use on pantyhose and toothbrush
6. Ballpoint Pen
7. Zip
8. Slinky
9. Rubic cube
10. Sticky notes
11. Contact lenses
12. Two-ply toilet paper 
13. Dunning-Krugger effect (Wikipedia)
14. Bubble wrap 
15. Elastic rope 
16. Jenga (Wikipedia) 
17. Twister game 
18. Crosswords
19. Controlled studies in medicine 
20. Fastener