Lutz - Trains in Germany

Oneputt

We went to Dresden by train yesterday, buying the ticket no problem, on the return journey the ticket collector gave us a look like we were naughty children as we hadn't validated the ticket.  When we brought the ticket there was a blue machine which I take
is the validator.  When do you carry out the prosess, as soon as ticket purchased or on the return leg of the journey.  Thanks in advance