Here's why:

* The Triangle Inequality Theorem: This theorem states that the sum of any two sides of a triangle must be greater than the third side.

* Let's check:

* 4 + 8 = 12 > 11 (This works)

* 4 + 11 = 15 > 8 (This works)

* 8 + 11 = 19 < 20 (This does not work!)

Since the sum of the two shorter sides (8 and 11) is not greater than the longest side (20), these lengths cannot form a triangle.