Yes, when using "firstly, secondly, etc." in a sentence, it is common to separate them with a comma. This helps to clarify the sequence of the points being made and makes the writing easier to follow. For example, "Firstly, we need to gather the necessary materials. Secondly, we should prepare the workspace." **

Full-face helmets provide more protection for the face and jaw in the event of a crash, making them better for more aggressive and high-speed cycling such as downhill mountain biking or BMX racing. However, for general cycling, half-shell helmets are often preferred due to their lighter weight, better ventilation, and less restrictive feel. The choice between the two depends on the type of cycling and the level of risk involved. **

Yes, it is necessary to have bicycle helmets. Wearing a helmet while riding a bicycle can significantly reduce the risk of head injury in the event of a crash or fall. Helmets provide crucial protection for the brain and skull, and can potentially save lives. It is important for both children and adults to wear helmets while cycling to ensure their safety on the road. **

Mangas can vary in their content, and while some may contain brutal images such as severed body parts, not all mangas are brutal. It ultimately depends on the specific genre and storyline of the manga. As for the second question, it seems to have been cut off. If you could provide the complete question, I would be happy to help answer it. **

Yes, you may install LED lights on your bicycle. LED lights are a popular choice for bike lights because they are energy-efficient, long-lasting, and provide bright illumination. Adding LED lights to your bicycle can improve your visibility to others on the road, especially in low-light conditions, and help enhance your safety while riding. Just be sure to check your local regulations to ensure that the installation and use of LED lights on bicycles comply with any specific laws or requirements in your area. **

Yes, there are bicycle accessories that make noise. For example, some bicycles have bells or horns that can be used to alert others of your presence. Additionally, some riders may attach accessories such as spoke clackers or playing cards to their wheel spokes to create a clicking or rattling sound as they ride. These accessories can be useful for safety and visibility, especially in crowded or noisy environments. **

Bicycle traffic lights have two red lights to make them more visible to cyclists. The two red lights are placed side by side, making it easier for cyclists to see the signal from a distance and in their peripheral vision. This helps to ensure that cyclists can clearly see when they need to stop, promoting safety at intersections. Additionally, the two red lights can help prevent confusion or misinterpretation of the signal, as it is clear that both lights being red means the cyclist must stop. **

When the bicycle brake locks the wheel, the wheel stops spinning and skids along the ground. This can cause the rider to lose control and potentially crash. Additionally, the tire tread can wear down quickly and the tire may even blow out from the friction. It's important for riders to be aware of how to properly use their brakes to avoid locking up the wheels. **