Right. Here comes the answers!

Stress Balls:

Remove 2 balls from the 8.
[1st Try] Weigh 3 balls on each side of the balance scale.
If it is the same, then
[2nd Try] Weigh the 2 balls you have taken aside to find out which is the heavier ball.

Bingo!

If it is not the same, then remove the 3 balls that are lighter. Keep 1 ball from the heavier 3 and
[2nd Try] Weight the remainding 2 balls.
If it is the same, then the ball you've kept aside is the heavier one. If not, the scale will show.

There you have it!

More stress leh:

Mark the balls properly so that you can tell which bag they came from later.

Remove 1 ball from Bag 1, 2 balls from Bag 2, 3 balls from Bag 3 etc... and weigh the 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45 balls on the weighing scale. If it is 2 grams more that it should be, then Bag 2 is the one with the heavier balls. If it's 3 grams more, then it would be Bag 3 and so forth.

Hope you guys had fun!