Leonardo da Vinci made illustrations of solids for Luca Pacioli's 1509 book The Divine Proportion.
These are the first illustrations of polhedra ever in the form of "solid edges", allowing one to see through to the structure. However, it is not clear whether Leonardo invented this new form or whether he was simply drawing from "life" a series of wooden models with solid edges which Pacioli designed.
One of these depicted solids is the tetrahedron or triangular pyramid with 4 equilateral triangles as faces.

drag the grey points

Drag the grey points of the tetrahedron on the left to the graph on the right, so that to top of the tetrahedron is the central point of the graph.

The tetrahedron is one of 5 Platonic graphs. These solids have congruent vertices, faces, edges and angles.
In the planar drawing and the graph you can clearly see that a tetrahedron has got 4 vertices, 6 edges and 4 faces.
This follows Euler's formula. Euler stated that convex polyhedra, with v the number of vertices, e the number of edges and f the number of faces, always follow the rule v - e + f = 2.
For a tetrahedron we get 4 - 6 + 4 = 2.