Thing is, folks don't suck eggs much any more; but back in the day, it was a great discovery that you good get good stuff out of an egg just making a little hole in it at either end without breaking it.
If you were real good you could slip the enmpty shell back into the row, and mystify the cook.

So kids would just nacherly get excited about this new skill and try to show off by showing Granma how to do it.

'COurse she learned about it 60 good years earlier. Hence the expression.

A