Certain areas of mathematics initially appear "hand wavy" as they often start with intuitive concepts before being rigorously defined. Popularized accounts of mathematical ideas tend to simplify complex topics, making them more accessible but less precise. Historical examples, such as Fourier series and Euler's solution to the Basel problem, illustrate how foundational concepts were initially viewed skeptically due to their lack of rigor. Over time, these areas evolve from informal to formalized frameworks as deeper understanding develops. This progression highlights the natural development of mathematical fields.