One of my favorite items of camping gear (which we brought on our recent motorcycle trip) is an MSR Whisperlite International camping stove. The "international" part refers to its fuel versatility: it runs on the clean "white gas" liquid fuel found in some American camping stores, and also on regular gasoline, kerosene, and diesel. The compatibility with gasoline makes it perfect for road trip camping, because (as long as the tank's not empty) it's always possible to siphon gasoline from the vehicle.
So, how do you siphon gasoline? The basic principle involves a suction (to pull the liquid into a tube) and gravity (to continue bringing it from one end to the other without a constant pumping). You could buy a siphon pump to do this. I've done that in a past but that takes up precious space.
You could also simply buy plastic tubing - available at our favorite local hardware store - and inhale the gasoline to start a suction. But then - as I learned on day 2 of our trip - you get a mouthful of extremely noxious gasoline fumes, which makes you lightheaded, and leaves a taste in your mouth that's very hard to get out. (I thought wrongly that with only liquid coming through the tube, the fumes would be avoided, but was very wrong.)
So, not wanting to inhale any more high-octane fuel (gasoline in Argentina, like most non-U.S. countries I've seen, starts at 95, not 87 like in the U.S.), I looked around our campsite for ways to fashion a siphon pump. The two necessary components (besides the plastic tubing) were right in front of me: a soda bottle (previously used for water), and duct tape.
Put one end of the plastic tubing in the bottle, and seal it onto the top with duct tape. Then compress the bottle so there's very little air left in it. Put the other end of the tubing in the gas tank, and (using one hand to make sure the seal is airtight) try to expand the bottle. This will start a suction from the tank into the bottle. Hold the bottle below the tank so gravity helps, and you should get a nice flow going. When you have enough, pull the tube out of the gas tank, and make sure that gravity empties the tube into the bottle. Take off the duct tape cap and you can fill your stove. Just don't use that soda bottle for water again...