@DexerMang
One grape hidden among three cups: [1] [2] [3], you pick cup [1]
If the grape is in cup [1], you lose if you change.
If the grape is in cup [2], GM opens cup[3], you win if you change.
If the grape is in cup [3], GM opens cup[2], you win if you change.
That's 2/3 odds for winning if...