Explaining black holes

Tonight, my 7 year old daughter was asking me about the black hole recently discovered at the center of our galaxy. It’s great that she is taking an interest in science. I want to make sure that my daughters both have an appreciation for math and science. First off, this will help them to be good problem solvers, no matter what they do. Second, the job markets of their generation will likely favor those with these skills. The other part of this, is that I had some difficulty explaining black holes to her. I know the basics, but started to trip up when she wanted more details.