The Rambam says that the guardians of orphans are not allowed to give tzedaka from their possessions because there is no end to the amount of tzedaka that one can give and therefore we are concerned that they will completely deplete their funds.
This is ODD! Give 10 percent like everyone else? How can we tell them to give NOTHING??
The Lubavitcher Rebbe discusses this at length here and to give credit where it is due - I heard about this sicha in an inspiring talk delivered on the 20th yahrtzeit of the Rebbe [for those who hold that he died...:-). For those who don't - the anniversary of the day that the non-believers THINK he died, or the anniversary of the day when people stopped sending questions directly to the Rebbe and started putting them in random volumes of his collected letters. I have a friend who will swear that he had a miraculous answer given through this method] by my beloved friend, Rabbi Shlomo Einhorn Shlita, the dean of Yavne in Los Angeles [he starts talking at about the 35 minute mark].
[Note: Rabbi Einhorn quotes answers in the name of Rav Shimon Shkop and Rav Gustman while the Rebbe in the sicha seems to quote from a much earlier source - Rav Yaakov Emden, in his She-elas Yaavetz. Incidentally, he was niftar the very same year the the U.S. was born. Can we say ובא השמש וזרח השמש - the sun of one tzadik doesn't set until another is born. Look at how many tzadikim America has produced over the last almost 240 years:-). The Rebbe in the sicha also continues with a further analysis not mentioned by my beloved friend but I am sure that due to time and other considerations, he kept it brief. The most important thing is to start off your day with MODEH ANI!!:-)].