Bum-bum, bum-bum, the sound of one heart beating, silently screaming the truth under all the lies.
Toph could feel their heartbeats, thus whittle out the truth. She could decipher all their lies.
She knows Katara lies about wanting Aang, because her heartbeat tells the truth. Bu-bu-bu-bum, bu-bu-bu-bum, the sound of Katara's heart beating as Aang comes near. Toph feels and Toph knows. She knows that Katara lies.
She knows that Aang tells the truth, because a heart never lies. Bu-dum, bu-dum. Aang's heartbeat stays steady. Toph feels and Toph knows. She knows that Aang tells the truth.
She knows that Sokka loves Suki. Truly and deeply, because you can't fool Toph. Bu-du-dum, bu-du-dum. Sokka's feelings stay true. Toph feels and Toph knows. She knows that Sokka loves Suki.
Toph knows when they lie. She always knows the truth, no matter what they say. When Toph feels a heartbeat, she knows what they don't say. You can't trick a heart.
For being blind, Toph is all-seeing. As she feels heartbeats, she hears the truth ring in her mind.
'She's lying.'
'He's scared.'
'They're asleep.'
She can know all about them, just by feeling their heartbeat. She can know their feelings. She can get inside their heads. She can know everything from a heartbeat. And with every beat, all she can think is…
'Thank God they can't feel mine.'