I'm not a native English speaker but I was wondering the same a while ago and I found this thread on wordreference: http://forum.wordreference.com/threads/is-easier-for-to-me-difference-in-meaning.2336732/ To cut it short: 1) Geometry was easy as pie "to" my younger sister = she thought geometry was easy (but maybe changed her opinion later when she had to solve particularly tough problems) 2) Geometry was easy as pie "for" my younger sister = she was really good at solving geometrical problems Hopefully some native speakers will turn up and say what they think.

In this one (and only this one) situation, you could make the joke/pun "Geometry was as easy as pi to/for my younger sister." (as to your actual question, either "to" or "for" will be fine.)

hi my friend i will tell you a simple explanation of to and for that will lead you to understand the phrase better we use "to" In These Cases: -Destination -time(what time is it) -Distance --Comparing -Giving -Motive -Reason and we use "for"In These Cases: -Benefits -Period of time(this one differ from the above because wwe ask for the period not the exacte time like"i'll be out for an hour") -Schedule -Agree with -Doing something to help someone -Motive -Reason and so the difference between the first and the second phrase is that in the first phrase where you used "to" it expressed or refereed to the appearance of something like it was simple to her in her thinking or your thinking and as for the second when you used "for" you expressed or you compared like it was easy for her then others i will give you two examples so that you can get my idea here Adam is stupid but Jack looks smarter to me! Adam is smarter than Jack for me ! hope i helped i'm not a native speaker so if i was wrong or i missed something i hope a native speaker can correct it its always better to learn from mistakes but at least for me i think this is correct hope i helped!!

I would say...they are the exact same. No difference :) Both mean the younger sister thinks geometry is easy/is good at doing geometry.