Question: how to share functions definitions among more than one open worksheet?

Currently, I have some definitions of modules and or functions in one worksheet, say A.

To reduce clutter in A, I'd like to open second worksheet, say B, and invoke the functions defined in A in the B worksheet. This way I keep the definitions in one worksheet, and the tests/trying them, in another worksheet.  I do this all the time in Mathematica. It useful, as I like to keep one notebook for just definitions, and second one for testing and calling the functions.

I just found out, that a function defined in worksheet is not "seen" in worksheet B.  I looked at options and settings, googled a little, but not able to find an option to allow me to do this. I tried Tools->General->Share one engine among all documents. Clicked Apply. But that still did not do it.  From worksheet B, it tells me it does know the function I am trying to call. Only in worksheet A it can see the function.

Is there an option to allow one to have the function definition be seen on all open worksheets?

I know I could save the functions as mpl files, and then load them in the second worksheet. But during development, the first method is easier.