Why does your body hurt when you grow (e.g. growing pains)?

Muscular: Growing pains is related to the muscle tenderness/cramps seen in some children who are very active during the day. They usually have a tighter muscle tone than others. It's not truly related to growth spurts. Make sure they drink plenty of water during the day and if this happens often start a little stretching/yoga as part of bedtime routine. http://www.webmd.com/children/guide/growing-pains