Food offered to the Buddhas should not be bad or rotten on the pretext that no one will eat it. It is good to offer one's own food before eating it. Since the main purpose of making offerings is to reduce avarice, one should do so without a trace of regret. The Buddha recommended that avaricious people should initially accustom their minds to giving, by giving something from one hand to the other. Water is also commonly offered.
Water is pure.
Water can easily and honestly obtained and when offering it one can imagine washing away the miserliness of all sentient beings.