Want a flatter stomach, so you do hundreds of sit-ups and crunches to burn the belly fat right where it sits. Want toned arms, do arm exercises to melt the flab there. It is the entire premise of half the workout videos ever made: target the area, lose the fat there.
You cannot spot-reduce fat, and the science on this is clear. When your body burns fat for energy, it draws on fat stores from all over, in a pattern set largely by your genetics and hormones, not from whatever muscle you happen to be working. Doing sit-ups strengthens and builds the abdominal muscles underneath, which is genuinely useful, but it does not preferentially burn the fat lying on top of them; studies that had people exercise one limb intensively found the fat came off roughly evenly across the body, not from the worked area. To reveal those ab muscles you have to lower overall body fat through a calorie deficit and general activity, and the belly is often one of the last places to slim for many people. So the targeted-toning promise is a fitness-industry favourite that does not hold up. Train the muscle for strength, by all means. Just know the fat leaves on its own schedule, from wherever it likes.