What's makes the tide a gravitation pull

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.        

well, there are a lot of places we get information about earth from. like science classes, websites, books, newspapers, etc.