Say aliens have abducted you. After probing you for a while, they get bored and drop you off wherever they choose. You look around you and see nothing but forests and snow-capped mountain peaks. And the only instruments you have on you are your lucky lighter and a thermometer that they left in your . . . let's say your mouth. What vital data about your whereabouts can you get using just those tools?
For one thing, you can determine the temperature of the place, and that's very helpful, I'm sure, but you can also dabble in the art of hypsometry. Hypsometry is the ability to measure height. In this case, you're measuring your own height, with respect to sea level. Start by heading for those forests and gathering wood. You'll need it. You'll also need a broad leaf, or perhaps some paper cups that hikers, or other abductees, have left behind. Start a fire, keeping it very low. Suspend a cup, or a broad leaf, over the fire and fill it with water. We've seen that, as long as it's filled with water, it can't burn.
The water should begin to warm. Stick the thermometer in there, keeping the fire low. If the water doesn't boil, slowly build up the fire. Keep a careful eye on the thermometer, and mark the temperature at the exact point that the water boils. This is the key to finding out how high you are.
Water molecules are held together by hydrogen bonds. The electrons in a water molecule hug slightly closer to one side than the other. This leaves the molecule slightly positive on one side and slightly negative on the other. Since positive charges attract negative charges, the negative side of one molecule will attract the positive side of another, keeping water together. But the liquid needs some help. Salt dissolved in the water will allow it to stick closer together longer. As will added pressure from the outside. The atmosphere provides that pressure, but it provides less at different altitudes.
At sea level, the pressure of all the air, from the molecules resting on the ground to the ones dancing at the edge of space, are on the water, keeping it together. A few thousand feet up, not all the air molecules in the world are pressing down on top of it anymore, and it comes apart easier. Water in a vacuum boils instantly. Water at sea level boils at 100 degrees centigrade. For every thousand feet anyone climbs, the temperature that water boils at drops one degree. If the aliens dropped you a thousand feet up, your water boils at 99 degrees. If they dropped you on Everest, and your water boils at 70 degrees - you probably should have been a better sport about being probed.