Malcolm has a heart-to-heart with Reese and his feelings regarding to school. They then proceed to go over what Reese did during the day that could have resulted in the blood seperating in the test tubes.
In this clip, Malcolm and Stevie are goving over the scientific method to rule out all the variables that could have lead to their experiment results. They are figuring out the unknown process which caused the blood to seperate in the test tube.