A simple experiment can be build to demonstrate a possible contradiction between thinking theory and reality. Use two hand electrical light torches set upon a table and back to back. Switch them on simultaneously, and calculate the relative velocity of their opposite leaving flashes. ←c← O= =O →c→ ←2c→ Solution In this case, the relative velocity between two leaving flashes is twice the speed of light. A matter of discussion Well, now put yourself in a teacher position and try to explain to your students, how this could be possible? I leave this answer to you!