Leaving a water bottle in your car might seem completely harmless. But on a very sunny day, it could lead to a real disaster. How you ask? By concentrating the light into a beam intense enough to burn through materials such as seat coverings in your car. This is why this particular water bottle which was recently being sold in Russia is so dangerous. For the World Cup, the Russian business Holy Spring created a water bottle that was actually shaped like a football that could act like a convex lens. Due to its spherical shape, it can concentrate a large amount of light; enough that it could light a match or even burn a hole in a hardwood floor. What is a convex lens? Light is made up of photons which are particles that move in a straight line. One of the many tools used to change how these photons move is a convex lens, which focuses them to a point called the focus. Glasses follow this principle and redirect the light, so it goes towards retina for people who suffer from short sightedness for example. This same method is used to make the light focus in tools such as a magnifying glass. The focus distance is established by the shape and material composition. All the energy from the sun’s rays is then concentrated on this point, which is enough to heat up, and even set fire to the targeted material. This effect is also possible with all plastic bottles that are empty or contain a transparent liquid such as water. The risk of starting a fire depends on the position of the water bottle in relation to the flammable material. If there is nothing to burn at this focal point, there is no risk. But if the conditions are right, leaving it for just a few minutes can be enough to cause an accident. A possible risk inside cars In a car, although the light has to travel through the car windows to reach the bottle, there is still enough heat to cause these types of accidents. Even through the glass, the sunlight can heat the seats up with more than 600 watts of energy per metre squared, the equivalent of a small electric heater. When this energy is concentrated on a certain spot, it can burn through plastic in a few seconds. Luckily, there is little chance that a water bottle can cause an actual fire in these conditions. Car seats are made of a common material, PVC, which naturally suppresses flames. But if a very flammable material was ever to be placed in the wrong place, it could cause significant damage. So, it’s probably better to never leave a water bottle out in the open in your car. As for the bottles made by Holy Spring, they have been recalled, despite all the excitement and attention they had from fans.