How do I explain masturbation and to a 4 year old?

Well my 4 year old niece walked in on me masturbating two nights ago. I was embarred to say the least. I tried avoiding her for the next few days so there would be no awkward tension and for the most part she tried avoiding me to. If we were to accidentally bump into each other we would each force a "hey" or "sup" then continue on our way. I feel it's time I explain to her what I was doing when she walked in on me so she can understand how natural it was. The only problem is I have no idea what to say to her. Someone please help.