The simple ones for some... I had to ask Kit about the bass clef notes. It's all relative! :P

4. The planets of the solar system. Poor Pluto.
6. Gears for driving

But that's all I can add at this glance.

8. One Two Three ... Nine Zero

7. Kilo Mega Giga Tera Peta Exa Zetta Yotta

15. Kingdom Please Come Over For Gay Sex
(Phyluum Class Order Family Genus Species)

16 is chess pieces.

1. colors of the rainbow
3. months
5. stellar spectral classes. (The letters stand for O B A F G K and M.)
6. ottermatic transmission gears

OMGWTFBBQ: I only got four of them.

Nice! I figured that'd be a hard one.

hahahah!!!!!!

1. color spectrum
2. musical scale - treble clef if memory serves, but it's been a while
3. months of the year
4. our newly-shrunk solar system
6. gear shift
8. base 9 counting - sneaky
10. bass clef, this time
15. biological classification
16. chess pieces

...that's all i got

13 stands for D C B A F G E, the rings of Saturn.
14 is orbitals; sharp, principle, diffuse, fundamental.

Wow, very good. I wasn't sure if someone would get electron shells and I really didn't expect anyone to get Saturn's rings. :)

1. Them Colors
3. Them Months
4. Them Planets
6. Them Gears
7. Them Bytes
8. Them Numbers
11. Them Elements
15. Them Classifications
18. Hot Simians Knitting Their Karp Teeth However Slowly Karp Teeth Knit Together Every Elemental Malted Nougat House Sailing Kitten Tails Knowingly Throughout

Well done. You spotted the hidden "them"s where I cleverly made the "T" invisible. Got quite a few of them, too.

Bravo! Wasn't sure if this one would be really easy or really hard. :)

1. Colors of the rainbow (Red orange yellow green blue indigo violet)
2. Lines of the treble clef
3. Months of the year
4. Non-dwarf planets of the solar system (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)
*5. I gave up and cheated with Google. Harvard Spectral Classification System for stars, apparently.
6. Automobile gears (park, reverse, neutral, drive, ?, low)
7. Large SI unit prefixes (kilo-, mega-, giga-, tera-, peta-, exa-, zetta-, yotta-)
8. Number keys in the top row on the keyboard (one thru nine, zero)
9. Vitamins
10. Lines of the bass clef
11. Elements of the Periodic Table
12. Presidents of the USA
*13. I thought these were the notes of the octave, but I didn't know why they went down from D instead of up from C, so I cheated and did a search. Rings of Saturn apparently?
14. Electron shells?
15. Taxonomical categories (Kingdom, phylum, class, order, family, genus, species)
16. Chess pieces (Pawn, rook, knight, bishop, queen, king)
17. Common units of volume (teaspoon, tablespoon, cup, pint, quart, gallon)
18. Parts of the body (Head, shoulders, knees & toes, knees & toes. Head, shoulders, knees & toes, knees & toes. Eyes & ears & mouth & nose. Head, shoulders, knees & toes, knees & toes.)

Ah, your usual quality showing, KT! Well done.

(And I believe the "I" is "Incline" on automatics.)

AGCAATTGCT/TCGTTAACGA

That sequence means you're allergic to shellfish, doesn't it?

Only the bottom-feeders.