A bowl of cereal? Just kidding. I always thought it was a ring.

Throughout out most of the song, it mentions Billie Joe McAllister jumping off the Tallahatchie Bridge. Only in the last stanza Bobby Gentry mentions something being thrown off. I think that it is an excellent song that tells the banality of life going on, even after a tragedy. I have passed over the Tallahatchie Bridge and is not high up at all.

Mama said to me, "Child, what's happened to your appetite?
I've been cookin' all mornin', and you haven't touched a single bite
That nice young preacher, Brother Taylor, dropped by today
Said he'd be pleased to have dinner on Sunday, oh, by the way
He said he saw a girl that looked a lot like you up on Choctaw Ridge
And she and Billie Joe was throwin' somethin' off the Tallahatchie Bridge"

Seems obvious to me. Brother Taylor got her pregnant and Billie Joe was her love. Power vs heart. She wasn't throwing something off the bridg with Billie Joe. She threw off Billie Joe. Brother Taylor blackmailed her into marrying him.

Quite awhile back I read somewhere that there were several more verses to the song than were recorded, one or two possibly suggesting that Billie Joe was involved with another girl and got her pregnant.

I would also hedge towards a fetus, and not a well developed one at that. Miscarriage or abortion done early sound like they could fit and drive a man who wants a family to jump off the bridge.

What was thrown off the bridge was an enigma, wrapped in a mystery.

It plausibly could have been a baby. There is no need to assume that the girl could not have hidden a pregnancy, especially if it ended pre-term (as with a miscarriage). In fact, the literature is full of examples of girls/young women giving birth where their parents were previously unaware of a pregnancy. A chunky physique, a loose-fitting dress...