Do you have any special ways to extend food so it will feed more people?

I put a tin of tuna packed in water in a bowl and was annoyed at all the liquid in it. I used to throw the juice out, but this time I added crumbled saltine crackers to it. The crackers absorbed the juice and the juice flavored the cracker, which became soft. I added the celery, relish, lemon juice and much less mayo than I usually add. Outcome: more tuna salad than one tin usually makes and, because of less mayo, fewer calories. No one noticed the difference.
Has anyone else extended some food to make it go farther?