Why is the use of mobile phone on board when the aircraft is refueling prohibited? ?

This is particularly dangerous because as the first person said cell phones have been known to spark causing fires. That is why it is said that you should not have your cell phone on you when pumping gas at the gas station. The electromagnetic waves can cause a spark causing a fire which can be dangerous. If someone is using their cell phone while refueling then it becomes a greater hazard because the gas tanks are larger and if a spark happens then you have a deadly explosion of a greater magnitude.