Anj, the lass who was in charge was talking about the core, ie our midde bit, including where your six-pack is (if you've got one - I haven't).
I managed an instant connection, which led to this:
Are apples good for the core?
Anj didn't seem to get it, but another lady present expressed her disgust at my attempt to alleviate everyone's pain.
It seems the joke was worse than the work-out.
Nothing daunted, when Anj stared getting us to do exercises for the abs, I had another go.
I recognised that 'abs' could be the first part of a word that could lend itself to being a punchline.
I mentally went through words beginning with 'abs' and quite quickly came up with what I thought was a good one.
Here's the resulting gag:
If you were to use your six-pack to power a yacht, could you call it abseiling?
There was laughter after I told that one, but the real sign of quality (if there is any in those two gags - you can decide) is that I shared them with my harshest critic, my wife, and she thougt they were both good.
I'm hoping that these examples of 'laughter-isation' will help you over time to pick up methods of joke-making which work for you.