If you play Bubble Bobble enough, you might begin to suspect that the collectible items that appear are essentially random.

But you'd be wrong.

Some of the more useful items are the bubbles that hold the letters that spell the word EXTEND. To get them, all you have to do is encase a lot of enemies in bubbles and maneuver them so that they all pop at the same time.

If you popped enough, when the next stage starts you'll notice that the lettered bubbles start pouring in!

In fact, you get two less than the number of enemies that you popped at once (pop 3 enemies, get 1 bubble; pop 5, get 3 bubbles; etc.)

And this works on every version of Bubble Bobble that I tried it on.