How do I do something completely, absolutely selfless?

I've been known for doing things just for the heck of it, just to make people happy. My dad and I were talking about it, and he gave me a challenge: to do something completely selfless.

The challenge in this is that if I do something good, and I know that I did it, I get a gratifying feeling from it. Even if I sacrificed something of my own just for the happiness of someone else, it still wouldn't be completely selfless. For example, if you gave a well-played Pokemon game to a kid that you would much rather keep to yourself, but give anyways because that kid might enjoy it, you feel terrible for giving away something you love but you still get that good giving feeling.

I conclude that there is no way you can not get a giving feeling after giving, unless you don't know that you are giving.

So, reddit, how do I give without knowing I've given? Thank you in advance, you're all wonderful.