public void mouseClicked(MouseEvent e) {
			if( e.isAltDown() ) 
				centersOfGravity.add( new int[]{e.getX(), e.getY()});
			
			//panalElements
			for(PanalElement p : panalElems)
				if( p.contains(e.getPoint()) ) {
					if( p == gravityUp  )
						gravity += .1;
					if( p == gravityDown )
						gravity -= .1;
					if( p == pause )
						playPause();
				}
			
			//select a box
			for(Box b : boxes)
				if( b.contains(e.getPoint()) ) {
					selectedBox = b;
					int selectedPadding = selectedBoxElem.width/2-b.width/2;
					selectedRect = new Rectangle( selectedBoxElem.x+selectedPadding,
												  selectedBoxElem.y + selectedPadding,
												  b.length, b.length);
					break;
				}
		}