it is an artificial enclosure for keeping small house plants
The dog was left in a small terraium so he wouldn't run away
If there is sand or anything that they can swallow that's not food related like gravel or rocks.I use carpet you can cut it to different sizes as far as your tank (terraium) goes.
Because they like to be in packs and they think that they will become lonely and will not get another friend to play, sleep etc. with. especaly if they where the only one in the litter or the rest died. but you can cure that bye putting a divider in the cage a leaveing there for 1 or 2 months and you can periodicly take them out see if they fight of not and if they do sprit them with some water. and they should stop and if it doesent you can get a fish net and put it between there nose and it will separated them them put them back in the cage and take them out every week and try again and when they get along you can take the divider out. but don't take it apart for about a month so that if thay fight you can put it back in. you can use a terraium cover for your diveder and fasten it to the cage with so they cant push it down a kill each other. don't forget to visit gpguineapig.wetpaint.com if you have or want to get a guinea pig thank you in advance.