When you are pregnant 2. When you are old 3. When you are completely knackered and are neither old or pregnant but are feeling like both and would murder someone for a seat 4. These people are nose pickers. This is pretty repulsive at the best of times but some people seem to think they get a cloak of invisibility on the tube and just think this gives them carte blanche to pick away.
Problem is they probably do it at home, which is fine.
This is essential when you are travelling on the tube. If you are carrying a heavy bag or briefcase, it's a must to stand with this between your legs as it will aid your balance. Sometimes on really crowded trains you may find that you are not holding on to the famous dangly balls and you are being held upright purely by the weight of your fellow travellers. In cases like this, seriously try not to think about it but take up meditation instead and stand firmly trying to channel all of your weight into your heels. In the cases where it doesn't work be prepared to fall over when your human supports decide to leave the train.