Unfortunately, your answer is wrong.
A glass does not possess characteristics for sucking. The air encased by the glass exert certain pressure. The physical dimension of pressure always describes how strongly a body presses against another body. The air inside of the glass cools back down quickly if it is not heated up anymore by the flame. Thus, the pressure inside of the glass decreases. Outside of the glass and inside of our body there is a higher air-pressure than inside of the glass. Therefore, our skin is pushed into the glass.