Magic coins trick

The magician places three coins in one cup, then casts a magic spell and pours the coins into another cup. A miracle happens: there are only two coins in the glass, instead of three. The magician puts the coins back into the cup and pours them back into the first cup. Now there is only one coin in the cup, and the other two have disappeared without a trace. 

The secret of the trick:
The cups have a small depression that is absolutely invisible to the audience. For simplicity, these recesses are marked with ribs: if you tilt the glass so that the rib is at the bottom, the coin rolls a little into this recess and thus does not fall out of the cup.

Size of one cup: 7.5x3.5x3.5 cm.