class Explosion(pygame.sprite.Sprite):
    def __init__(self, center, size):
        pygame.sprite.Sprite.__init__(self)
        self.size = size
        self.image = explosion_anim[self.size][0]
        self.rect = self.image.get_rect()
        self.rect.center = center
        self.frame = 0                                         # set frame to 0 (game loop increases frame by 1)
        self.last_update = pygame.time.get_ticks()             # set last_update to gametime in miliseconds (since object initaited)
        self.frame_rate = 50

    def update(self):
        now = pygame.time.get_ticks()                          # set now to gametime in miliseconds (since update method ran)
        if now - self.last_update > self.frame_rate:           # if now - class attribute greater than 50
            self.last_update = now                             # update class attribute to now
            self.frame += 1                                    # add 1 to frame when update method is ran if conditional is met
            if self.frame == len(explosion_anim[self.size]):   # this part I do not understand, the length of what exactly
                self.kill()
            else:
                center = self.rect.center                           # find new center of which image? I'm not sure.
                self.image = explosion_anim[self.size][self.frame]  # this sets the image to which ever explosion we are in in the list.
                self.rect = self.image.get_rect()                   # find the rectangle of that image
                self.rect.center = center                           # find the center of that rectangle