how to make destructable items

hi everybody i need help with a script. i would like to make a script that makes objects able to be destroyed. to do this one of the functions of the script would be when my bullet hits the block model i want it to be replaced with another model witch will be the cube destroyed in its place but i am confused and lost on how i would make this script do this does any one know how to write this kind of script or where i could find one if this has already been done? thanks