Cut the top section off a clean, empty milk jug. Cut around the jug at a height just below the plastic handle. When complete, the resulting container should look like a small square bucket.
Measure the width of the base on one side of the plastic milk jug. Also measure the height from the tabletop to the top edge of the cut jug. On a piece of construction paper, transfer the measurements and cut out a rectangle with the same length-by-height dimensions.
Convert the construction paper rectangle into an isosceles triangle. The base dimension of the milk jug is the base dimension of the triangle. The height of the triangle is the height of the side of the milk jug. Cut the two long sides of the triangle so that they measure the same length, and meet at the center point of the top edge of the rectangle.
Overlay the triangle template onto each side of the milk jug, and trace the triangle pattern onto the jug. This template will form the sides of the pyramid.
Cut the jug so as to create the triangle shapes on each side of the milk jug. When cutting from the top edge towards the base of the jug, do not cut along the base, which would remove the side completely from the plastic container. Cut each side so that it is triangular shaped, and connected to the base.
Fold each of the triangular shaped side of the milk jug inward until the edges of the triangles meet with each other. Using the clear packaging tape, tape these corner seams together. Because each triangle is the same size, and they're connected to a square base, the four triangular shaped sides should meet perfectly at a single point and create a miniature pyramid.