How come some plums are bitter and others sweet?

i LOVE sweet plums and it's such a bummer to bite into a plum and it ends up being one of those bitter, sour ones. is there anything to tell the two apart? color or size or something?

How come some plums are bitter and others sweet?
I actually grow plums in my back yard. I had two separate trees (now i have just one). What i always noticed was that if i piked out the ones that were firm (not hard) and the darker ones, those were usually the sweeter ones. Of course you still have your different kinds of plums. There are the dark purple ones, Maroon colored ones, and the white ones. The white ones are the sweetest I've EVER tasted.
Reply:no one knows................