Mar 2:22 And no man putteth new wine into old bottles: else the new wine doth burst the bottles, and the wine is spilled, and the bottles will be marred: but new wine must be put into new bottles.
This verse was confusing me when I first studied it as I didn't understand how Bottles would burst. However I recall see a show on TV where a man explained the bottles of the time. Here is a link to show you about those:
Bota bag - Wikipedia, the free encyclopedia
What the link shows is a bottle made of animal skins. The show I watched explained how wine placed in a new bottle would cause the bottle to stretch due to the fermintation process of the wine causing a gas to build and expand the bottle. However, once the bottles had been used for some time thru this process it would cause the bottle to have reached a point where it couldn't expand much more without bursting. Therefore, you would only put older wine (that had already been thru the fermenting process) into the older bottles and only put the new wine into the newer bottles.
I hope this was helpful in giving some background on the verse.
Paul