Grease, bones, and small pieces of metal 

I feel like the eggshells don't hurt. The article i post said you should used them either.I'm pretty sure you aren't supposed to put eggshells in there breh
scraps of food followed by dish liquid and a run of hot water
Clean that mfer with bleach once a week
Scraps,
Eggshells,
chunks of fat from when you cut up some meat,
unedible parts of the carrot or onions when you cut up veggies, things like that.
Unfinished food scrapings from your plate.
basically anything that would make your garbage smell.
but obviously not tossing whole slices of uneaten pizza in there or something crazy like that.
also avoid anything that would break the shyt like Chicken Bones
Whenever i want to clean it i put in some crushed ice from the fridge and soap and turn it on.
you have no plants at all in your yard or house? eggshells are good for them.I feel like the eggshells don't hurt. The article i post said you should used them either.
but it said specifically not to use a large amount of eggshells. ANd the reasoning was because they don't decompose quickly and build up.
So as long as its just a small amount and you do plenty of other stuff in between i think its alright.
I put ice in there periodically to help clean it out and break off any build up.