The lightning bolt spell creates a 100-foot long, 5-foot wide line of lightning. How tall is this line? Does a creature above the line, not on the ground, take damage because it is still, technically, within the line? The Line entry on PHB 206 isn't helpful, saying nothing about its height.
A line extends from its point of origin in a straight path up to its length and covers an area defined by its width. A line’s point of origin is not included in the line’s area of effect, unless you decide otherwise.