Newbie: Why do you say I can't keep my goldfish in a bowl? Everyone else does, and theirs are fine.
Fishkeeper: Yes, they seem fine for a while, until they die after 2 or so years. This might seem ages, but goldfish can easily live over 20 years. You can't keep a golfish in a bowl because:
These fish grow to a large size. Keeping them in a small bowl might seem okay when they're little, but as they grow they need more room to swim. Put yourself in their position... how would you like to spend your short life pacing back and foward in a closet?
Goldfish DO NOT grow to the size of their tank/bowl! A common myth is that if you place a GF in a bowl it will keep growing until it is a maximum size to be comfortable in that bowl. Lie. While the outside may stay small, the fishs' internal organs continue to grow until the fish dies. This is called stunting, and is a very nasty thing.
There simply is not enough water in a bowl, even a 'large' one, to hold enough oxygen for any fish to thrive.
Newbie: But the LFS guy said I could!
Fishkeeper: Simply because 'the LFS guy said so' doesn't mean he's right. His job is to make money. I know this is cliche, but if the LFS guy told you to take a bath in petrol, wash your hair with beer and jump into your fireplace, would you?