1. Toph knows that Sokka has measured the exact angle he would need to throw his boomerang in order to knock the Avatar right in the back of the head. Just in case he would ever need to use it.
2. Sokka and Toph know that before they left that tiny village at the base of the volcano, Sokka quietly paid a secret visit to Aunt Wu while Aang and Katara were packing up Appa. She had repeated her nonsense about him leading a miserable life, and then warned him that he would have to protect the hearts of the one's he loved. He swore that day he would protect Aang and Katara to his death, if it meant protecting their hearts.
3. Sokka and Toph know that when they prepare for their waterbending lessons, Katara can no longer look at Aang as he strips his clothes with ease in front of her. She looks away, blushing lightly, but Aang never sees, being all tangled up in his shirt, and Katara doesn't seem to notice that what was once natural has turned awkward.
4. Sokka and Toph know that Katara is slowly starting to lose her taste for meat. It's not very obvious yet, and she's not doing it on purpose, but with each meal she is selecting more fruit for her plate and less of whatever it is Sokka has caught.
5. Sokka and Toph know that Katara cannot sleep during the full moon. They also know that Aang will wake up with her every time, no matter how tired he is from training, and they will go to the nearest source of water and explore their bending powers - and each other.
They believe that Sokka and Toph sleep happily through these nights. Sokka knows Toph is listening silently to the ground for any intruders that the two flirting teenagers might miss. Toph knows Sokka is stroking his boomerang possessively, mentally adjusting his angle to account for lying on the ground when it's thrown.
6. Sokka and Toph know that when Zuko and Iroh first approached Aang with the intention of making some sort of deal – teaching the Avatar firebending in exchange for safety in numbers – Aang's first thoughts were not of whether their offer was a trap, or if it was safe to let them so near to him. They know his first thoughts were of Katara, and how the addition of firebenders would affect her.
7. Sokka and Toph know that while Aang still feels badly for burning Katara those months ago, he is secretly relieved to be learning firebending despite the fact that he swore he'd never try again. They know this makes him feel so guilty it's as if he's burned her again.
8. Sokka and Toph know that while Katara still remembers how badly her hands hurt that day Aang burned her, she's secretly grateful that it brought out her talent for Healing. They know she's excited for Aang to complete his training and reach his full potential.
9. Sokka and Toph know what Aang was yelling in his sleep on that awful night that started with a simple nightmare and ended with the four of them scrambling through the thick smoke while Iroh and Zuko attempted to control the blaze.
Aang blamed the smoke for his sore throat, but Sokka and Toph thought it had something to do with the fact that he was screaming Katara's name when he shot the powerful blast of fire in his sleep, setting the dry timber alight.
10. Sokka and Toph know that Aang is mentally warring with himself. Half of him is pleased that Katara and Zuko are getting along and that his teachers will cooperate with each other as they train him. The other half of him grows more possessive as each day passes and the smiles Zuko gives Katara appear to become genuine and Katara's thinly veiled hostility loses its sharpness.
11. Sokka and Toph know that Katara's heart is completely and utterly broken that day they find the destroyed Water Tribe ship off the coast of the earth kingdom port. Sokka's heart is broken too – once when he sees the destruction of the ship and the red stains that have dried purple on the blue interior, and again when his devastated sister withdraws from everyone and refuses to go to Sokka or Aang for comfort. They know she wants to appear strong.
12. Sokka and Toph know that Aang has broken through Katara's sadness the following full moon when they leave the campsite for the nearby stream and stay away for so long that even Sokka dozes off waiting for their return. Zuko and Iroh rose too late to find Aang and Katara curled up together, one of his arms protectively around her. Toph successfully wrestled the boomerang from Sokka's hand.
13. Sokka and Toph know why Zuko wouldn't speak to Aang or Katara for a week after their last run in with Azula, and they know it has something to do with the fact that Aang and Katara ganged up on the knife thrower, distracting her with Katara's waterbending before Aang blew her head first into a nearby tree.
Even the warrior in Sokka was nauseated by the cracking sound her head made, and she hit the tree so hard that Toph saw it clearly from across the field they were fighting in.
14. Sokka and Toph know that they have to watch Katara's back, because no matter how hard she fights Azula and her gang, her mind is not there in the battle but miles east at the temple where Aang, Zuko, and Iroh have gone to confront Lord Ozai. They know that if something should happen to Katara and Aang survives his battle with Ozai, Aang will never forgive himself.
15. Sokka and Toph know that as the world celebrates the end of the war, Aang's job is not over. Peace must be made between the four nations, and soon they will all face the choice: return home to their families, or continue traveling with the Avatar as he calms the world.
They know that the question will not even enter Katara's mind, because she's told him from the beginning that she will never leave him. (They also know that it's not going to be much of a choice for them either, but they will at least pretend to think about it.)
16. Sokka and Toph know about one close call that Aang was never even slightly aware of – then again, he wouldn't expect an attempt on his life from someone he considered family.
But the moon was so bright and full, the music was so slow and sensual, and Aang and Katara were dancing so closely together. Toph caught him at the last minute – thankfully Sokka was stomping around so hard she saw exactly what he was planning on doing. The Avatar was saved only by the sharp jab to the elbow on Toph's part and the right turn in the music that took Aang away from the flying boomerang and put the son of a poor cabbage merchant in its path.
17. Sokka and Toph know that Aang and Katara haven't heard the latest criticism; the newest attempt by Aang's very few enemies to discredit his peaceful message. People were claiming that the Avatar should marry a non bender – someone who would have a better chance of providing him with an airbender. The fact that he's marrying a waterbender because he loves her apparently means that he doesn't love the world.
They know that when Aang and Katara hear this, Aang will go into a sulk, wondering what he did to deserve this, and Katara will spend a week mothering him and trying to explain that it's not him that they're against, it's the peace and love and justice that he stands for that they hate.
18. Sokka and Toph know what the baby will be months before it is born. Toph knows the instant Aang helps Katara step down from Appa – she can, after all, sense anything that makes a vibration.
19. Sokka and Toph know the real reason Katara is constantly writing in that blank book.
It's not just so she can record her adventures for her family to pass down from generation to generation as she claims – it's also for the tiny part of her that quietly despairs Aang moving on to another life and forgetting. Years from now the book could fall into the new Avatar's hands, and he'll remember.
20. Sokka knows how it will end. (Toph may know too, but she's gone to wherever souls go when the body they are in dies.) They are leaving in the opposite order that they entered his life: Zuko first, then Toph, and now it is obvious that Aang is growing weaker and is preparing for his next life. Katara has not acknowledged it, but Sokka knows it will end the same way it began: he and his sister alone in the South Pole, helping their tribe – and each other – survive.