How would you explain to an athlete why damaged joint ligaments and cartilages are so slow to heal following an injury?

They have little to no direct blood supply.

Explanation: “Ligaments and cartilage injuries are very common injuries for people who practice sports. Damaged joint ligaments and cartilage are slow to heal because these structures don't have a significant blood supply, which brings necessary fluid and nutrients to the injured place. These nutrients and fluids are essential for damage repairing. If ligaments and cartilage become injured, their limited blood and nerve supply result in a longer recovery and healing period.”

burning of wood and coal

explanation:

the panel also concluded there's a better than 95 percent probability that human-produced greenhouse gases such as carbon dioxide, methane and nitrous oxide have caused much of the observed increase in earth's temperatures over the past 50 years.