The kindest thing I have ever done is to refuse to participate in threatening people with the massive destruction of the H-bomb. If you or the human species don’t survive, the only continuing kindness possible would be to give other species the gift of survival and normal life. But they wouldn’t be aware of any human act of kindness, only that they were living with their personal problems. It takes a human consciousness of past, present, and future and the relationships of various actions to recognize a kind act. Animals can recognize friendly behavior but it seems unlikely that they recognize intentional kindness.
To grant continuing life to a living being is an act of kindness and that is why I don’t kill flies in my house, but instead, I catch them and put them outside. “It helps one develop the habit of kindness towards others, even flies, and toward yourself. When you are intentionally kind to flies you will be kinder to your own self and be a happier person. It is easier to practice being kind to flies than to people because people are more distracting, and we forget what we are trying to do.” Tiny acts of kindness are readily available, like putting flies outside, and doing them lets you practice frequently until a great opportunity comes along and you can do a kind thing that is needed right now, instantly, reflexively.
In daily life, people come along and one of the kindest acts is to hear what they say, and the proof of that is to respond appropriately to what they actually say.