Why do my knees crack when I bend down?

Like say if I'm picking something up, my knees [sometimes only 1 of them] cracks D: Anyone know why? My dad has problems with his knees also [they crack whenever he stretches his legs out as well as when he bends] so it might be hereditary? It doesn't hurt most of the time, but on Mondays, I [sometimes, not as much as I used to when I was younger, which was every week] have really really sore knees/legs , like I nearly start crying with the pain:( Although I told my parents about this & they said it was just growing pains, and that it'd stop as I grew. It did stop an awful lot, [as I said it's only sometimes now] but still. Also, a few [2 maybe?] years ago I started cracking my knuckles [fingers & teos lol] so maybe this contributes to it/ [Altho I doubt it lol]. Sorry it's long but can any1 help with my problem please? Thanks a lot:)