Postman delivering opened mail

Have to agree, my mum pulled this on me a few times when I lived at home....

Dont accuse but next time it happens tell her how odd it is that its the second time she's been able to read your post!!
 
Or you could post some 'bait' letters with your name on the outside, and 'Dear Mum, thanks for opening my post for me' on the inside!
 
Go with the bait letters, it'll be worth the cost of a stamp to see your mums face!!