My back tire got flat. If I pump it up now, it can hold the air for about 30 minutes to an hour before it's totally flat.
I took out the inner-tube yesterday, pump in a bit air and put it into water, trying to find a hole but couldn't find anything.
Could it be that the tire is too old and I must replace it?
EDIT: thanks to all your response. Beside soap water, I found out hot water is really helpful and that was what I used to fix my tire. There was a very tiny hole on the tube that I couldn't find when I used room temperature water.