question of the evening

adam looked at me last night holding a yellow plastic cup in his hand and said, "is this your cup?" i said yes, of course...but thought to myself, "two of us live here with a small person who is not yet able to drink from a cup like that. now, please, tell me....if it's not yours, and you think it's not mine, and the only other person living here drinks from a Gerber sippy cup....who's cup do you think it is?