Coins and coffee

       This morning when I cleaned up my desk and I found that there's some coins on my desk.I wonder why there's some coins cause I remember all of my money supposed to be stayed in my wallet or other people's wallet.
      Well, just now my housemate knocked my door and come in asked for instant coffee. I gave her, she paid me 1 dollar and said : it is nice, you give me instant coffee, i paid for you, i don't have to go down and spend $1.50 for a cup of coffee. After that she said that when I went back to Malaysia, she took my instant coffee, and put money on my desk.As my roommate never told me that, no wonder my desk got  unknown coins!