|MadSci Network: Biophysics|
Thanks for your question,
Cows are waterproof! Their skin, like ours acts as a barrier to water and microbes as well as a complex sensory system used to detect pain, heat and pressure.
What we call leather is in fact the skin from a cow that has gone through a long series of treatments such as curing, soaking, painting, liming, fleshing, de-liming, bating, pickling, degreasing and finally tanning. See the link below to find out what all these words mean. You will find that leather can be tanned in many different ways and this will make them more or less susceptible to water damage.
Your shoes probably are made of leather as they are able to stretch somewhat with use. Also the shop assistant would have made sure that you had ample room for at least 3 months growth. Assuming that you are of high school age I would not be surprised if your feet have now stopped growing or have slowed down and this might account for why you can still have the same shoes after several years. If you are growing, take care! shoes that are too small can cause your feet to change shape and deform.
Try the links in the MadSci Library for more information on Biophysics.