Agree with all that's been said. You can sing. You just haven't found your voice yet. The more you do it, the easier it will become to pick up and hold pitch and tune. Start out by singing whatever you like and tell people it's an "interesting" harmony.

I've observed something similar with children taught to read music and play instruments at school. Without the music in front of them, they fumble to play anything. My daughter was brought up sleeping under pub tables at singarounds (blame the parents) and also did music at school - she has the wonderful ability to both play by ear and read music. It just comes naturally to her, as does singing.

McGrath of Harlow is absolutely right - all children deserve to be brought up on music and stories. As Mr. Steve Knightley says, "Without our stories or our songs, how will we know where we came from?"