Hi, Altan. The usual saying is "Θεέ (μου), πώς κρατάς τα κεραμίδια ξεκάρφωτα;". We say this when we hear something outrageous and it's similar to saying "how is the world still standing when everything should be falling apart?"
Regarding the crab, the full expression is: εδώ σε θέλω κάβουρα, να περπατάς στα κάρβουνα, meaning "let's see how you can do/ explain this difficult thing".