Until I met Jonas, I thought there were only two ways of breaking into a banana (without using tools or making a big mess)*. You either make a surgical incision near the flower end with your front teeth, or grab the stem and apply brute force.**
Just like the sausage, the banana has two ends. How can there be a third method?
* I bet someone from India will inform us that 79 other methods are described in a lesser known second volume of Kama Sutra (a.k.a. the Banana Sutra).
** For those who like vivid discussions here at instructables, I could label the first "the evolutionist's method" and the second "the creationist's method". Why? Well, the evolutionist would argue that we have co-evolved with the banana, developing suitable incisors in the process. The creationist, however, would claim that the stem was designed [by god] for us to grab and tear at. Which is the better? Which is p.c.? Are the labels justified? Correct? Alternative (non-p.c.) labels? (Admit that I just saved your next coffee break at work.)
Step 1: Jonas' Method
The only remaining place to open the banan is the centre. The method is quick, spectacular and produces a clean split. Best of all, it is almost as simple as it looks.
Step 2: Pulling?
No, simply pulling on the banana does not work very well. The "flesh" is soft and weak, so we only have to consider the forces in the skin. The illustration above shows the stress at the fracture plane, shortly before the fracture takes place. Since we pull straight, the stress will be about equal to the fracture stress everywhere
Due to this stress distribution, the total force will have to be quite large. To be able to pull that hard, you must grasp the banana with quite some pressure, or else you will slip. This is why this strategy often fails. The banana is often mushed before it breaks. This is ok if you plan to feed an infant with it, but that usually not what you aimed for.
Step 3: Bending?
What about bending the banana? This time the stress distribution is quite different. There is tension on the lower half of the banana in the picture above and compression on the upper half. This is the drawback of this strategy. Although less effort is needed, the skin usually fails on the compression side first. The banana will not snap off, but bend. More baby food is produced.
Step 4: Tearing!
By combining pulling with bending it's possible to concentrate the tension to one side, minimise squeeze forces, and avoid compression on the opposite side. I call it tearing since it is akin to what you do when you tear a piece of paper. You must concentrate the tension to a small part since paper is quite strong when loaded evenly.
Step 5: The Trick
It may be possible to pull and bend simultaneously, with no further ado, but I prefer to increase my batting average with a small trick. If I close my hands really close together, I can let the fingers on the outside (top) take the compression. That is, since I use the fingers as a hinge, I can get away with mostly bending, just pulling a little.
Step 6: Practice Makes Full
The skin of green bananas is quite tough, that of old ones with black spots much weaker. Old bananas are much softer though, so they are easy to mush by mistake. The best combination of weak skin and reasonably firm flesh is to be found in bananas which are on the verge of becoming their first black spots. Start training on those. Later on, you can try your luck on black and green ones.
Learning to keep a straight face is the toughest part...