Why does my laptop overheat when i play games?

my computer has directx11 and plenty of memory and stuff and when i put it on my lap i use a book as a surface so i dont cover the fan yet when ever i play a game it over heats. One time it over heated to the point whee i couldnt keep my hand on it because it was burning me. I swear i could cook an eff on that thing. I have a pavillion dv7 and it happens when i play halflife 2.