what's the difference between jelly and jam joke

The joke you’re looking for is:

“What’s the difference between jelly and jam?”
“I can’t jelly my d*** up your a**.”

This is a crude, sexual double‑entendre that plays on “jam” as both the fruit spread and the verb “to jam (force) something in,” while “jelly” doesn’t naturally work as a verb in that way.

A cleaner, family‑friendly spin people sometimes use in forums is something like:

“What’s the difference between jelly and jam?
You can’t pump up the jelly.”

That version riffs on the phrase “pump up the jam” (from the song) and swaps “jam” for “jelly” for a silly, non‑explicit twist.

For context, the actual food difference is:

• Jam: made from crushed or pureed fruit plus sugar, with bits of fruit in it.

• Jelly: made from strained fruit juice plus sugar, so it’s clear, smooth, and more jiggly/firm.

Bottom note: Information gathered from public forums or data available on the internet and portrayed here.