Buying a ticket at the station. Look up at the map and look to see where you want to go - then insert the amount of money shown under your destination. If you are not sure then just buy the cheapest ticket and pay the difference when you arrive at your destination.
I remember trying to do that when I was living in London - I got the cheapest ticket and wanted to pay the difference but instead ended up paying a poxy lousy 20 pound fine. London Underground - honestly - they will take every opportunity to take money from you and find every excuse. Every time I go back to the UK to see my folks, I find that ticket prices have doubled but the service seems to get worse.
