Fishing is an activity that's easily accessible and popular with people of all ages and experience, and there is a wide variety of techniques. Any experienced fisher will have his own tools and techniques to gauge good fishing conditions, but a popular one is a fishing barometer. This owes some of its popularity to the fact that it is easy to make at home and even easier to read.
The materials needed to construct a homemade fishing barometers are easy to find. Take a standard 1 qt. mason jar and fill it about 2/3 full with water. Then take an empty glass bottle, such as a soda or beer bottle, and place it upside down inside the water-filled jar. After a couple days, you will notice that the amount of water that accumulates inside the bottle varies. At this point your barometer is ready to gauge fishing conditions.
The water inside the glass bottle is your indicator. Depending on the pressure, the water level inside of it will rise and fall. When the bottle's water level is higher, barometric pressure is higher. Conversely, a lower water level indicates lower pressure. According to common beliefs, the best fishing times occur during higher pressure or falling pressure. So when when most of the water is in the jar, the fish are less apt to bite. Instead, you would find better conditions when there is more water inside the bottle or when the water in the bottle has just begun to recede. This falling level of water signifies falling pressure, which is usually a sign that a weather front is approaching. Typically this atmosphere will have the fish biting a bit more than normal. It is important to keep in mind, though, that using barometric pressure to judge fishing conditions will produce spotty results, as different types of fish thrive in different atmospheric conditions.