Answer:
As Demons or Devils are fallen angels, they rarely allow themselves to seen. But one can call up a demon by habitually doing wrong, thereby leaving oneself open to further promptings of a demon. Their visitations are done little by little so that the individual is unaware that they are under the influence of a demon. Their evil deeds then become a natural part of their lives, giving no thought to the consequences of their actions. They then become completely under the influence of the demon which possesses them. If a child often does wrong things, someone might say "They have the devil in them". This of course is not true as demons cannot influence little children.