1) One of the reviewers has used this for a hamster. I think it would be safe though. There's also some photos of hamsters eating it.
2) I'm sure it comes with instructions. I think you just sprinkle seeds over a pot or tray. I think it comes with the soil stuff it grows on. I would water it a couple of times and leave it on a windowsill.
3) Maybe try it out at playtime or in a playpen and see Sweep's reaction. If he's not too bothered, it would be fine in the cage. If he goes crazy for it, give it as a treat.
4) No idea! Maybe test it out for us
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