Poop. Lots and lots of poop...and water everywhere. Wet bedding, poop, and that special odor under the heat lamp...Feeling traumatized...
They're cute though. My kids like the mallard breeds best-khaki campbell's are their favorite. The kids imprint them out of the incubator and play "bite my finger" through the bars of the cage for about a month before "duck mode" sets in. They also let them fight for who gets to sit in their hand. Kids are 3 and 5. They INSIST on raising all ducklings they imprint in the house.
Specific questions are easier to answer. Also, if you just do a search on this site you'll probably find what you need already posted.
If they quack loudly they want something, they need water to swim in and to drink, dont feed them spinach, if one has no balance put it in a sock under heat for a day, dont put them down unless it is ABSOLUTELY NECESSARY