Which of the following is NOT considered an ignition source in vehicles?

Rainwater is not considered an ignition source in vehicles because it does not produce heat or energy that could ignite flammable materials. Ignition sources are typically characterized by their ability to generate enough heat to cause a fire or to produce sparks that can lead to combustion.

Hot surfaces, such as exhaust manifolds or catalytic converters, can become extremely hot and are capable of igniting nearby combustible materials. Internal combustion engines produce heat during operation and can also create sparks, making them a potential ignition source. Mechanical sparks can occur from various components during vehicle operation or from electrical systems, further supporting their role as ignition sources.

In contrast, rainwater, while it may affect electrical systems or create slippery surfaces, does not possess the property of generating heat or combustion in the context of initiating a fire. Therefore, it is excluded from the list of ignition sources in vehicles.