回复：一道难题

Fill the upper glass with water first --- so that the bottom of the glass closely attaches to the coin's surface (assume the coin's surface is smooth neough).
Then fill the gap between the upper glass and lower one, use the empty pen pipe to suck out the water in the upper glass gradually, do this until the upper glass is floating to the top the lower one, the coin should be sticked to the bottom of the upper glass and thus got out.