Y'all people should use common sense, give Duolingo some slack!! ;)) plus it's capital P so you know it's a name right here
Pedro is a name (names are capitalized). That name means stone indeed, but in Portuguese, the word is "A pedra". You cannot speak pedro meaning stone (And there's no girl called Pedra either)
But, what do I say when bidding my pet rock farewell? or throwing a pebble off a cliff?
To me i'm saying goodbye to a donkey (If you read boys life you would know why)