A strange and awful event I met in my dorm

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Today ,my dorm happened a strange thing that make my roommates and me nervous and frigetened. The thing happened like this: this afternoon, before my two roommates and me went to have supper,we exchanged a 20-litre bucket of drinking water on the drinking machine, but when I was back to dorm, the third roommate ask me: were you exchange the bucket?"I replied:"Yes".then, she said:"but the bucket is empty." I didn't see the bucket is full or not,but I remember we exchange just now,so I said:" the bucket must full water" she said:"It's empty now" . so I turn back to show her the bucket is full,but what make me shock is the bucket is empty. We sure we have exchange 20-litre water on the drinking machine,and the third roommate also sure while we left,our dorm's door was closed and no one come to our dorm and the floor was dry .But waht make us strange is where the 20-litre water has gone.