Why do we breathe?
At the simplest level, we breathe in order to take in oxygen and remove carbon dioxide from the blood stream. Breathing enables us to mix the gas in our lungs (which equilibrates with the blood, oxygen diffusing in and carbon dioxide out) with the air and so replenish these concentration gradients down which these two gases diffuse between the lung and circulation. Oxygen is crucial to the catabolism of fuels such as glucose in order to create cellular energy from aerobic respiration, and so is crucial to cellular survival - with oxygen stores only maintaining the brain for about 2 minutes. It is also important to remove carbon dioxide because of its acidic properties - reacting with water to create carbonic acid. To maintain the blood at its pH of 7.4, carbon dioxide must be breathed out or the blood will become acidic - which can cause pain and even cause heart arrhythmias at extremes.
Is it true you can't prepare for an Oxford interview?
Sort of. Whilst it can be said that as their interview process aims to 'test your way of thinking' you can't prepare learned answers to questions, however knowing the type of questions that they ask in addition what preparation to do is important in order to perform well.
If I cut my finger, why doesn't pressing the cut stop it from bleeding?
In order to understand this, it's important to understand the anatomy of the blood supply to the fingers which actually runs on the sides of each digit - meaning that in order to stop your finger from bleeding you need to hold the sides of your finger in order to compress the arteries from which the blood is leaking.