Answer:
Flexibility is defined as “the ability of a muscle or muscle groups to lengthen passively through a range of motion”, whereas mobility is the “ability of a joint to move actively through a range of motion”.
Step-by-step explanation:
Keyword is what exactly are you doing. If you're basically stretching your arms without adding any sort of movement, that's flexing, but if you're moving your arms around like doing any sort of thing (like reaching to a cup) then it becomes mobility.