I would say shape, hardness, and texture
When a Stickleback fish feels threatened, it will flare out its pair of pelvic spines and three dorsal spines, making it difficult for predators to swallow them.
Answer:
Radio frequency is short for RF, which is a kind of high frequency alternating current electromagnetic wave with frequency range from 3k hz-3k GHz
Because the frequency is high, so the molecular movement speed is fast, and the physical explanation of the heat of the object 100 is its internal molecular movement speed, the use of radio frequency energy to make the molecular high speed movement of course will achieve the purpose of heating
Bursa - A small sac that lubricates the area around a joint where friction is most likely to occur
ex. elbow, knee, or shoulder
