This shows how to find the orthocenter of an
acute triangle by folding.
This video uses Tabula by
The orthocenter of a triangle is the
intersection of the three altitudes.
Mathematics of paper folding
The art of origami or paper folding has received a considerable amount of mathematical study. Fields of interest include a given paper model's flat-foldability (whether the model can be flattened without damaging it) and the use of paper folds to solve mathematical equations.
Read more at
Wikipedia, Mathematics of paper folding.