OK, if you have followed the blog during its not so long history, the answer to this last visual perception test was quite obvious. Of course, the deformed one. One of the balloons weighted more than it should, and consequently, the stick to which it was stuck buckled.
That's the kind of deformation I want to explain you briefly today.
And what's buckling, then? It's a kind of deformation, so to say. Nothing new, just told you that.
Let's go a little further: it happens to things under compression. And still one step further: you'd rather it wouldn't happen.
Make a little experiment. Take a cane (a stick, a ruler, anything that's thin will do), and press it lightly against the floor. That way, you're applying compression to it. Now, increase slowly the pressure you apply. While you press, it will remain straight. Until...all of a sudden, you will notice that the stick bends out!
That's buckling. It happens so unexpectedly. All at once, it buckles, and it no longer stands any load.
And that's why it is a very dangerous problem. When it happens, there's no way to keep it under control.
Imagine that one of the columns of your house disappeared. It could be a disaster. If one of the columns of your house buckled, you'd face a quite similar problem. It would be almost the same as it wasn't there.
Therefore, structures have to be designed not to buckle. Conversely to the traffic sign, "do not buckle, it's the law!".