Monsters can see player resistances

If monsters see you resist something, generally elemental or magical
attack, or if they see you reflect an attack, they learn that and
will adjust their attack accordingly.

Originally from SporkHack, but this version comes via EvilHack with
some minor changes.

src/fountain.c

@@ -542,9 +542,10 @@ drinksink(void)
         break;
     case 2:
         You("take a sip of scalding hot %s.", hliquid("water"));
-        if (Fire_resistance)
+        if (Fire_resistance) {
             pline("It seems quite tasty.");
-        else
+            monstseesu(M_SEEN_FIRE);
+        } else
             losehp(rnd(6), "sipping boiling water", KILLED_BY);
         /* boiling water burns considered fire damage */
         break;