Due to Lebesgue and Brouwer. If an 
-dimensional figure is covered in any way by sufficiently small
subregions, then there will exist points which belong to at least 
of these subareas. Moreover, it is always possible to find
a covering by arbitrarily small regions for which no point will belong to more than
regions.
Tiling Theorem
See also
Tessellation, TilingExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Tiling Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/TilingTheorem.html