Place a white card behind the straw so that its water level can be seen easily. Try putting your bottle in warm places, and cool places. Watch the water levels when you hold it by the freezer, or in the sun. If your thermometer works properly, the water in the straw gets higher when it's hotter, and lower when it's cooler. Everything is made of molecules.
Molecules are kind of like building blocks, or lego.
But there are always small spaces between the molecules. When something is hot, the space in between the molecules gets bigger, and the molecules spread out. When something is cold, the space in between the molecules gets smaller, and the molecules get closer together. That's why the water in your thermometer goes up when it's hot, and down when it's cold. The water expands when it experiences warmer temperatures, and the water contracts when it experiences cooler temperatures.