What is the full definition of: mitochondria

Johnny rode his bike to a friend's house 4 blocks down the street in his neighborhood. he immediately rode back home once he realized his friend was not able to play. what was his displacement for the total bike ride? how did you determine this? what could we use as a reference point to determine he was in motion during his bike ride? why can you use it as a reference point

What causes darkening of the skin as melanin production increases

Whats one way that rocks do not follow the typical rock cycle pathway?