How did he know?

[a_svirn]

No: HPFGUIDX 146921

> Carol:
> While we don't know for sure that TR didn't make any Horcruxes while
> he was still at school, I tend to think that KJ is correct; he found
> out how to do it from Grindelwald before 1945, his last year (or
> half-year) at school. <snip>

a_svirn:
But WHY would Grindenwald share this kind of knowledge with a 
potential rival? Take Voldemort, for instance, would he teach anyone 
how to become immortal? Would he show any eager aspirant to the world 
domination a way to actually achieve it? Not likely I'll bet. He would 
much rather eliminated the threat while it's still in the bud.