How come they differentiate the blood group of each person

There are two main antigens that form around our red blood cells, they are known as A and B. So, if your blood type AB, then you have both the A and B antigens. Type O lacks the A and B antigens, so the antibodies of a type O patient would attack A or B blood from a transfusion.

it also supports my hypothesis. if you rest first you can squeeze the clothespin more times. from the data i collected the average of the eight people concluded exercise makes your muscles more fatigued and less able to squeeze the clothespin compared to resting before squeezing.