def serve_handler(self, button):
"""
Callback when serve button was pressed.
"""
button_pos = list(button.to_window(*button.pos))
button_pos[0] -= 100
beer = Beer(lane=button.lane)
button.lane.puck_area.add_widget(beer)
button.lane.beers.append(beer)
beer.pos = beer.to_local(button_pos[0], 15)
