The title is self-explanatory. (Translation: I'm too lazy to write an intro.)

1. You tell them you don't want to be friends but they keep talking to you.
2. They keep asking you personal questions (especially questions about your relationships).
3. They have your phone number, address, or email even though you're just acquaintances.
4. You often see them around your workplace/neighborhood even though you know they don't live/work there.
5. You start seeing them around town often, during grocery shopping or something (exception: You live in a small town.)
6. They're always staring at you. One way to test this is to look bored, and then quickly glance at the clock and look back at them. (If they are watching you, they will automatically follow your eye movements and you will catch them staring.)
7. Their phone is always facing you when they're "texting" (They're probably secretly taking pictures of you)
8. They know lots of information about you that you've never told them.
9. Your friends ask about him/her.
10. He/she asks your friends about you frequently.
11. He/she calls you regularly even though you're just casual friends. They hang-up often or leave silent messages.
12. They send you texts that don't say anything (blank spaces. And they're wasting your phone
bill.)
13. They send you inappropriate gifts (such as flowers/chocolates) even when you've told them you're not interested. The gifts get weirder. Sometimes stalkers also give gifts anonymously.
14. The stalker magically shows up whenever you need help. (This doesn't apply if they work/live in the same neighborhood as you)
15. They offer to let you borrow their stuff.
16. They get extremely depressed when you don't show affection back.
17. They always have to butt in your conversations.
18. They always embarrass you in front of others.
19. They only talk to you when no one else is nearby.
20. They're always invading your personal space (standing WAY too close).

Disclaimer: This is not 100% accurate. The person could just really like you, or they're just plain weird. Or you're the one who's being paranoid.
Warning: If you find out that the person is stalking you, record all the weird stuff they've done to you, including the date, time, place and description (in case you decide to sue them later). If they  still don't get the message, threaten to call the police. Try to have friends/colleagues/family/witnesses around you at all times.

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