Doc simply smiled as he spoke.

"Not at all a joke. I have sent out 3 small versions of this bus - only in real working conditions- to prove that a time machine actually can be built. Time travel actually exists! All that is needed a proof from actual humans. That is where you come in.

Each of you will be allowed to keep a certain amount of ...... treasures from where you travel but I need you to bring back certain things as proof of your journey. I have the Mother Bus carefully programed and it will take you places that you never even thought of visiting." 

While Doc was talking. Mac opened another door then began to load the belongings into the bus through a special door which weighed the belongings or rejected them. Fortunately the chute accepted all of the belongings. Mac then closed the chute and opened the bus door.