I remember hearing that when I was a kid; finally thought of an answer one day:
Our earthly body is mostly water, with lots of minerals, and some other ingredients in suspension. Dry us out sufficiently, and we become a solid collection like "the dust of the ground (Gen. 2:7)".
So, considering we are the Lord's creation, and keeping in mind that we can only attain salvation (and be lifted up to Heaven) by accepting the call of the Lord; then, God really did make a rock so big He could not lift it. The rock is, you..."upon this rock I will build my church."
How do you like that?