Background: A baby is born with zones of cartilage in its bones that allow growth of the bones. These areas, called growth plates , allow the bones to grow longer as the child grows. By the time the child reaches an age of about 18 to 25 years, all of the cartilage in the growth plate has been replaced by bone. This stops the bone from growing any longer. Even though bones stop growing in length in early adulthood, they can continue to increase in thickness throughout life. This thickening occurs in response to strain from increased muscle activity and from weight-lifting exercises.

Situation: Two brothers, Tom, aged 26, and Paul, aged 16, started weight-lifting to gain more muscle mass. Three years later they were 20-25 lbs. heavier.

Question: Which brother had active growth cartilages?
Answer: Paul

Situation: Two brothers, Tom, aged 26, and Paul, aged 16, started weight-lifting to gain more muscle mass. Three years later they were 20-25 lbs. heavier.

Question: Which brother didn't have active growth cartilages?
Answer: Tom

Situation: Two brothers, Tom, aged 26, and Paul, aged 16 were into sports. Tom started weight-lifting to gain more muscle mass. Three years later he was 20-25 lbs. heavier. Paul started track and field, he liked running.

Question: Which brother had all of the cartilage in the growth plate replaced by bone?
Answer: Tom

Situation: Two brothers, Tom, aged 26, and Paul, aged 16 were into sports. Tom started weight-lifting to gain more muscle mass. Three years later he was 20-25 lbs. heavier. Paul started track and field, he liked running.

Question: Which brother didn't have all of the cartilage in the growth plate replaced by bone?
Answer: Paul

Situation: Two brothers, Tom, aged 26, and Paul, aged 16 were into sports. Tom started weight-lifting to gain more muscle mass. Three years later he was 20-25 lbs. heavier. Paul started track and field, he liked running.

Question: Which brother had thicker bones after three years?
Answer: Tom

Situation: Two brothers, Tom, aged 26, and Paul, aged 16 were into sports. Tom started weight-lifting to gain more muscle mass. Three years later he was 20-25 lbs. heavier. Paul started track and field, he liked running.

Question: Which brother had thinner bones after three years?
Answer: Paul

Situation: Two brothers, Tom, aged 26, and Paul, aged 16 were into sports. Tom started weight-lifting to gain more muscle mass. Three years later he was 20-25 lbs. heavier. Paul started track and field, he liked running.

Question: Which brother's bones had stopped growing?
Answer: Tom

Situation: Two brothers, Tom, aged 26, and Paul, aged 16 were into sports. Tom started weight-lifting to gain more muscle mass. Three years later he was 20-25 lbs. heavier. Paul started track and field, he liked running.

Question: Which brother's bones hadn't stopped growing?
Answer: Paul

Situation: Two brothers, Tom, aged 26, and Paul, aged 16 were into sports. Tom started weight-lifting to gain more muscle mass. Three years later he was 20-25 lbs. heavier. Paul started track and field, he liked running.

Question: Which brother's bones responded to strain from increased muscle activity?
Answer: Tom

Situation: Two brothers, Tom, aged 26, and Paul, aged 16 were into sports. Tom started weight-lifting to gain more muscle mass. Three years later he was 20-25 lbs. heavier. Paul started track and field, he liked running.

Question: Which brother's bones didn't respond to strain from increased muscle activity?
Answer: Paul