The farmer will give you one bag, which you use to power the hovercraft hidden in the mill attic.
(see related question)
The gold coin in Astro-knights is in the fountain. You will see something glittering in the water, and if you pick it up, it will be the coin. Use it to pay your admission to the Mordred museum.
you cant you have to have the owl (hint: it is under-ground the yellow bales of hay)
pick up the fuel rod and connet and put the rolling things on 56x52
The only things you do in the castle dungeon (under the library) are pick up the cheese and throw the switch. (see related question)
just walk pat it and it will pick it up
If you can't pick up the cheese or throw the switch in the dungeon, you could try going out and coming back in. Otherwise, reboot Poptropica in your browser. You can't complete the island without doing this.
Go to the Princess's Maid's room (first floor left in the castle) and feed it to the mechanical mouse. Also pick up the secret message. (see related question)
Google "Astro Knight Island Walkthrough" on the stuff that comes up, pick a youtube video. That was easiest for me.
Using Merlin, you pick up the bombs that Mordred throws out, then fly up and drop them on the green orb when they first turn red. The explosion will damage the suit.
The robot itself is dropping them (why - when you can fly away from them - nobody knows). Pick up one before it turns red, and carry it over the robot. When it flashes red, drop it on the robot. You have to do this twice.
Strangely, he has only two uses. The first is to pick up the Fuel Rod for you, and the second is in the final battle of the game, when he picks up bombs and drops them. (see the related questions)
Pick up the moldy cheese, and flip the switch on the wall (it opens a back entrance into the cell). (NOTE : once you do these, you cannot get back into the dungeon through the library. If you cannot find another way in, see the related question.)