Go and Catch a Falling Star

I like this poem. It's used in my favorite book; Howl's Moving Castle by Diana Wynne Jones. I still haven't quite figured it out. 

Go and Catch a Falling Star
by John Donne

Go and catch a falling star,
Get with child a mandrake root,
Tell me where all past years are,
Or who cleft the devil's foot,
Teach me to hear mermaids singing,
Or to keep off envy's stinging,
And find
What wind
Serves to advance an honest mind.

If thou be'st born to strange sights,
Things invisible to see,
Ride ten thousand days and nights,
Till age snow white hairs on thee,
Thou, when thou return'st, wilt tell me,
All strange wonders that befell thee,
And swear,
No where
Lives a woman true, and fair.

If thou find'st one, let me know,
Such a pilgrimage were sweet;
Yet do not, I would not go,
Though at next door we might meet;
Though she were true, when you met her,
At last, till you write your letter,
Yet she
Will be
False, ere I come, to two, or three.

If my Grandma

If my grandma switched from cleaning juices to mandrake juice, dragon blood, and vaporized toad dust, she would be a witch.

Mandrakes

I researched mandrakes and mandragoras on wikipedia, and I was amazed. Every single bit of the plant is poisonous. The root looks like a person, and it was used in witchcraft and alchemy. Not to mention all the weird legends about them...