the.com/simmering
Cooking's polite simmer just below a boil, and emotions doing the exact same thing.
means Heating liquid gently so it bubbles softly without breaking into a full boil, or by extension, tension building quietly beneath the surface.
from From the older English word simperen, to bubble, an onomatopoeia for the soft hiss of a pot kept just under boiling.
Sweet spotRoughly 185 to 200 degrees Fahrenheit.
Visual cueSmall bubbles, not a rolling boil.
Figurative useAnger and feuds both simmer convincingly.