Sforno on Numbers 21:8/ספורנו על במדבר כ״א:ח׳

1עשה לך שרף, the snake should be constructed of material which is associated with שרפה, burning. The association with “burning one’s tongue” should be uppermost in their minds when contemplating their sin which had been that they allowed their tongues to utter thoughts that angered G’d. They would repent when looking at a snake called שרף, something which “burns” (transitively).