It's harder than you think. You are bringing a living being into your home, with an individual history, personality, and collection of strengths and weaknesses. It's also more rewarding than you think, for exactly those same reasons. Every child makes themselves a different space in your home and in your heart, with their own nickname and funny catch phrase, and every child will be remembered in a different way, for different things. Some may be more difficult than others, some may refuse to sleep, some may be cuter than heck, but you'll be amazed at your own ability to make room for, and in turn, let go of every child that comes through your home. Being a foster parent isn't something I'd recommend to the faint of heart, but for those with large and durable hearts, it is an incomparable experience that has the ability to truly change children's lives, as well as your own.
