function toggleSink(id) {
	var sink = document.getElementById('sink'+id);	
	
	if( sink.style.visibility == 'hidden' || sink.style.visibility == '' ) {
		sink.style.visibility = 'visible';
		return true;
	}
	else if( sink.style.visibility == 'visible' ) {
		sink.style.visibility = 'hidden';
		return true;
	}
}