The Torah discusses two ways a dangerous pit might come into existence: someone might open an existing pit that was previously covered, or someone might dig a brand-new one. In (Exodus 21:33), the verse says "if a man open a pit," and shortly after it discusses digging. The Mekhilta links these two actions together through a legal principle that illuminates both.
Opening is compared to digging, and digging is compared to opening. This mutual comparison allows the rabbis to transfer legal rules between the two cases, filling in gaps that neither verse addresses on its own.
From the comparison of opening to digging, we learn about permission. Just as one who opens a previously covered pit on his own property — with proper authorization — is exempt from liability if someone falls in, so too one who digs a new pit with permission is exempt. Permission is the key factor, not the method of creating the hazard.
From the comparison of digging to opening, we learn about size. Digging a pit creates a hazard only when it reaches a specific depth — the Talmud later establishes this as ten handbreadths, deep enough to kill an animal that falls in. By comparing opening to digging, the same size requirement applies to uncovering an existing pit. Simply lifting a cover off a shallow depression does not create legal liability. The pit must be deep enough to pose a genuine danger.
This technique of mutual analogy — where two related cases each teach something about the other — is one of the Mekhilta's most sophisticated legal tools. Neither verse alone tells the full story. Together, they create a complete and coherent law of hazardous pits.