One half of every pair does more work
Ask how BIG a mouse is and nobody blinks. Ask how SMALL it is and you have already given away your opinion. The same lopsidedness runs through the whole set: we ask how OLD a baby is, how FAST a snail goes, how HIGH the step is, and never the other way round.
Linguists call the default member of a pair the unmarked one, and children tend to pick it up first. It is a useful thing to notice with a young child, because deliberately using the quieter half — how short is it, how young are you — is a cheap way to give a word some exercise.
Some opposites are a dial, others are a switch
HOT and COLD sit at either end of a slider. Things can be quite hot, a bit cold or lukewarm in the middle, and not hot certainly does not mean cold. OPEN and SHUT is a switch: if a door is not open, it is shut, and there is no third option to argue about.
This is why some questions can be haggled over and others cannot. Is it BIG? Depends what you are comparing it with. Is the door open? No it is not. DAY and NIGHT look like a switch until somebody mentions dusk, and WET and DRY behaves like a dial for a towel and a switch for a phone.
One word in this grid is its own opposite
Hold FAST. Stuck fast. Fast asleep. A fast colour is one that will not wash out. All of those mean firmly fixed, which is the older sense — and the opposite of moving quickly, which is what the same four letters mean everywhere else in this puzzle.
The two meanings are related. Running fast once meant running firmly or vigorously, and the speed gradually took over. English has a small pile of these self-cancelling words: you dust a cake and you dust a shelf, and when ten people left the room it is anyone's guess whether they departed or stayed.
The opposites English forgot to build
Plenty of pairs are made by bolting a prefix on the front — kind and unkind, tidy and untidy — a pattern to point out to a child, because it turns one known word into two.
The system has leaks, though. We describe people as dishevelled but never as shevelled, as disgruntled but almost never as gruntled, as unkempt but not kempt — although kempt was once a real word meaning combed. Ruthless has outlived ruth, which meant pity. Somewhere along the way the cheerful halves were dropped and only the complaints survived.
Pairs to argue about
The grid holds a dozen matched sets: BIG and SMALL, HOT and COLD, FAST and SLOW, DAY and NIGHT, OPEN and SHUT, HARD and SOFT, HIGH and LOW, HAPPY and SAD, LIGHT and DARK, WET and DRY, OLD and NEW, LOUD and QUIET.
Two of them are quietly double-booked. The opposite of HARD is SOFT for a cushion but easy for a sum, and the opposite of LIGHT is DARK for a room but heavy for a bag. Spotting that a word can face two different directions depending on what it is sitting next to is an advanced idea, and a five-year-old can get there in about a minute with the right question.