the liquid now comes. use the marker to mark the outside of the bottle at the spot where the straw goes from being filled with liquid to being empty (i.e. the line where the liquid stops filling the straw). this line shows the current temperature where the bottle is at the moment.
move the bottle to different locations where the temperature is likely to differ. notice how the liquid either rises or sinks down in the straw. in warmer temperatures, the liquid will rise. in cooler temperatures, it will sink. try placing the bottle in direct sunlight, on a porch outside on a warm day, or in the refrigerator to see 