Yes, underhand serves are allowed in professional tennis matches. While they are less common and often seen as unconventional, there is no rule against using an underhand serve. In fact, some players have used underhand serves to surprise their opponents and gain an advantage. However, underhand serves are generally considered less powerful and can be riskier to use in high-stakes matches. **

The volleyball serve may not go over the net due to various reasons such as improper technique, lack of power, or hitting the ball at the wrong angle. Serving requires a combination of timing, strength, and precision to ensure the ball clears the net and lands in the opponent's court. Players need to practice and refine their serving skills to consistently execute successful serves. **

Birdsong is primarily caused by male birds singing to attract mates and establish territories. The songs are a way for birds to communicate with each other, whether it's to defend their territory, signal their presence, or attract a mate. The songs are also influenced by the bird's genetics, environment, and learning from other birds. Overall, birdsong serves multiple purposes in the bird's life cycle and is a complex behavior that has evolved over time. **

Friendzoning can be caused by a lack of romantic attraction from one person towards another. This can be due to various reasons such as differing interests, values, or simply not feeling a romantic connection. Sometimes, one person may see the other as a better fit for a platonic friendship rather than a romantic relationship. Communication and honesty about feelings and intentions are important in navigating these situations. **

Cancer is caused by genetic mutations that lead to uncontrolled cell growth and division. These mutations can be inherited or acquired due to exposure to carcinogens such as tobacco smoke, ultraviolet radiation, or certain chemicals. Additionally, factors like age, diet, and lifestyle choices can also contribute to the development of cancer. Ultimately, cancer is a complex disease with multiple contributing factors that can vary from person to person. **

Jealousy is often caused by feelings of insecurity, fear of losing someone or something important, low self-esteem, or past experiences of betrayal or abandonment. It can also stem from a lack of trust in oneself or in the relationship. Comparing oneself to others or feeling inadequate can also trigger feelings of jealousy. Ultimately, jealousy is a complex emotion that can be influenced by a variety of personal and relational factors. **

Nitrogen is primarily found in the Earth's atmosphere, making up about 78% of the air we breathe. It is created through a process called nitrogen fixation, where certain bacteria and other organisms convert nitrogen gas into a form that plants can use. Additionally, nitrogen can also be released into the environment through human activities such as the burning of fossil fuels and the use of nitrogen-based fertilizers in agriculture. **

Forgetfulness can be caused by a variety of factors, including stress, lack of sleep, aging, certain medications, medical conditions such as Alzheimer's disease or other forms of dementia, and lifestyle factors such as poor nutrition or excessive alcohol consumption. Additionally, distractions, multitasking, and information overload can also contribute to forgetfulness. **