to use one's head

(geometry) Euclidean distance - the distance between two points defined as the square root of the sum of the squares of the differences between the corresponding coordinates of the points; for example, in two-dimensional Euclidean geometry, the Euclidean distance between two points a = (aₓ, a_y) and b = (bₓ, b_y) is defined as:

(mathematics, geometry) Euclidian space - a space where the prinicples of Euclidean geometry apply.

those (referring to things distant from the speaker)