How to approach this situation

We bought my daughter a sandbox...she’s 20 months old. All she wants to do is throw all the sand out. For about 45 minutes, I tried calmly explaining that the sand stays in the sandbox and we can’t throw it out...if we do, we won’t have any more sand left. I also let her know that if she continues throwing it outside the sandbox, we’ll have to go inside. I probably repeated “don’t throw the sand outside the sand box” 40 times.

Eventually as I was scooping sand to put it back in the sandbox while she was continuing to dump it on the ground, I picked her up and brought her inside. I told her why and she cried a lot, of course.

How would other parents have handled this situation? It was so frustrating.

Also, what if she never “gets” it?? Do I just let her play and throw all the sand out until it’s gone?