I think there is a very subtle difference. hand and hand just refers to two hands not necessarily together.
hand in hand implies that the hands are connected or together.
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An arm is what you used to type the question. The hand is what I used to type this answer.
The difference between internal and external validity is in their nature. Internal validity indicates if a study depicts relation between two variables. External validity on the other hand generalizes the study of the variables.
difference between as on and as at
Equilateral deals with equal sides. Equiangular, on the other hand, deals with angles, Equal Angles.
Get a container that has markings for how much liquid is in it. Fill it half way with water, and write down how full it is. Then stick your hand in the water and measure the new water level. The difference between the two water levels is the volume of you hand.