What color are your knees?

We just finished Family Home Evening. We talked about the kids being able to ask us any question they wanted about the gospel. We sang, "I Am a Child of God," as our closing song. Then we knelt for our closing prayer and family prayer. Just as I was about to pray, Spencer says, I have a question. We shushed him because I was just about to pray. After the prayer Spencer again says, "I have a question." Ok, we said, go ahead.

Why in that song (I am a child of God) in the second verse does it say, "And so my knees are grey?"