My goal is to have a flatter belly. What I have learned over time is crunches aren’t as effective as these other exercises such as weight lifting, push-ups, and planks. Weight training is known to stimulate the production of mitochondria the energy engine in every cell that turns fat into fuel. If you have only 30 minutes in the gym, focus on weight-bearing exercises. However if you cant make it to the gym or being a member is just not in your budget have no fear. Thirty minutes of exercise in the convenient of your home is just as good. You just need a little space , a mat, some resistance bands and two pairs of free weights to begin with.