1. Flip a computer display screen upside down.
2. Drive the grammarian in your life crazy by tweaking their autocorrect.
3. Or if you reeeeeally want to up the ante...
4. Set their Facebook status privacy to "only me" so nobody will Like their stuff.
5. Turn their apps into cats.
6. Set a screenshot of their homescreen as wallpaper, and strategically move apps so they're repeatedly clicking on NOTHING.
7. Change all of the contacts in your friend's phone to the names of Harry Potter characters.
8. Rename all their files after Pokémon characters.
Or, you know, whatever floats their/your boat. This is hugely annoying to change back, but oh, the nostalgia.
9. Treat them to CAT FACTS.
10. If you work in an office (preferably where there are extra keyboards), switch around someone's keys.
11. Or block their mouse sensor with a Post-it.
12. Change the office printer's error message.
13. Pull a Nathan Fielder, if you're feeling brave.
14. Or try this one:
15. Cage-ify their Facebook.
16. Or, better yet, the entire internet.
17. Just go analog.
Simple, classic, elegant.