Problem with a flatmate

I have a problem that really bothers me every now and then: my flatmate uses or takes things that belong to me and she doesn’t even bother to  tell me that she did (usually she does this multiple times). This might seem stupid, but for me it’s a big deal and I don’t understand how a person can do this without shame or conscience… It’s not that those things are so important to me, but is mainly the gesture that infuriates me. After all it’s a matter of common sense!

If you have any ideas about how I should deal with this problem, please comment below! Help me thansform this problem in an opportunity!

Thank you!