Hi, im having trouble understanding why three vectors that lie in the same plane cant form a tetrahedron. If the plane is somewhat vertical or titlted will it not be possible for one vector to higher up than another so that you have a difference in height? Also, for three vectors to form a tetrahedron, can two of the vectors lie in the same plane? Additionally, if given 4 points, A,B,C,D and you want to check if the points all lie in the same plane. How come it works to check if the crossproduct between vectors AB and AC is perpendicular to vector AD? (i.e finding the crossproduct between vectors AB and AC and then finding the dot product between that and vector AD and seeing if it equals zero). Any help is appreciated. Please don't hesitate to ask if you need me to clear up on something regarding the questions.