I assume anyone reading this knows about Yoshi's Island or Yoshi's Island DS, so anyways, on to the trick.

When you hit an egg at a wall, it will change colour from green to yellow to red. Taking advantage of this mechanic, it is possible to throw an egg at a wall and then catch it with Yoshi's tongue. However, this requires fairly fast reflexes and good accuracy, so get a good distance between you and the wall. Exploiting the trick will get you six red eggs.

Why the heck do you ever want to use it? Having a constant supply of six red eggs means a ready source of stars. This makes it easier to get the 30 Star Points at the end of each level. I used it in Super Mario Advance 3, and I rarely had to use 10 or 20 Point Star Items.