Re: At the beach why do I see my friend's heart beating in his chest?

first, let's establish the fact that this was really your friend and not 
an alien-lookalike... I imagine your friend is probably about 10 to 12 
years old and slender. His left ventricle which is the pressure pump of 
the heart lies against the left anterior chest wall. It would not be 
unusual to see the heave of the heart at that point, particularly in a 
slender person who has an inverted chest - technically called pectus 
excavatum. The running around further accentuates this by delivering more 
blood with increased force in and out of the left ventricle at a greater 
rate. 

ps - if it literally leaps out of his chest I reccomend running the other 
way.