jądro

sail

n. core, kernel, pip; nucleus, positively charged heart of an atom (Physics); core of a cell containing DNA and RNA (Biology); core of the head of a comet (Astronomy); quintessence, bedrock; heart, hard core, unyielding part of a group; crux, root, gist, nitty-gritty, the main point, the main thrust

be at the bottom of something

get to the bottom of something

she is good at heart

the inner core of the Earth

n. core, center, heart; nucleus, kernel, central and most significant part of a whole; testicle, testis

Kern