A fixed mindset is when a person believes that they have limited potential, based on the innate skills and talents that they know they have. When all you commend a child for is her or his intelligence, this may box him or her in, preventing the child from exploring other strengths like being artistic or athletic.
Try instead to develop in them a growth mindset, wherein they see themselves as having the potential for growth and development. You can do this by acknowledging strategies, efforts and processes, and not just the outcome of what your child chooses to do.