THEREIN LIES THE ANSWER

An old nugget of 'strange but true' trivia offers the information that the word 'therein' contains within it ten words:

the / there / therein / he / her
here / herein / ere / rein / in

VitQ set out to widen the goalposts and find another ten:

re is a perfectly good word meaning either a note of the sol-fa scale ('a drop of golden sun', as the song says) or concerning in which sense it is not, as many think, an abbreviation of 'regarding'.

ein is the German word for a, one.

th is the name given to a rune symbol, a Y-shaped letter in Olde English script, or a Latin version of the Greek letter theta.

erein – a Basque word for to spread or sow, it is also the name of a publisher.

ther is a Chaucerian variant of the word there. Also known as a surname, as a short form of the first name Therajid, and as a part of personal names in Thailand and in the Middle East.

er is a word expressing a hesitation in speech.

ei is the German word for egg.

Herei was formerly a German model railway manufacturer. It is the Romanian name for the goddess Hera.

Erei is a mountainous region of Sicily.

rei is a Japanese word meaning zero (among other things). It is the Catalan word for king.