For example, fueling is done from barrels. Due to heavy passenger and freight loads, they typically don't carry any extra fuel beyond what is needed for the planned flight with appropriate reserves. On my flight in the Cessna 185 we had some minor changes, with unexpected passengers at our first stop who wanted to go to a landing strip that was on the way to our second destination. It would have been easy to brush off any fuel concerns by saying that it would hardly take anything extra, but I watched Al carefully stick the fuel tanks, take a hose to a shed, siphon some gas out of a barrel into a smaller container, and then climb up on the wing and pour the gas into the tank. Al said he was pretty sure we had enough fuel even with the unexpected stop, but just to make sure, he was going to put a little more in.