I'm new to bikes. I got a used bike that had all the tires and tubes broken so I had to change them all. I changed the tires and the tubes completely two days ago, then I went cycling the next days. Today I went to ride the bike to find out that the front tire is flat (not completely flat, since the tube has a lot of air). Is it just a bad luck? Or does that mean I have bad quality tires or tubes? Or is it just a matter of refilling the tube with air again? Or that I didn't close the valve of the tube correctly? I'm really surprised for this!