acf(c(0,1,2,3,4,5),plot=FALSE)
    Autocorrelations of series ‘c(0, 1, 2, 3, 4, 5)’, by lag
    
         0      1      2      3      4      5 
     1.000  0.500  0.057 -0.271 -0.429 -0.357 

Why as lag increases, the acf output becomes negative? My understanding is no matter what the lag is, the series is in general increasing, so the auto correlation should be positive. For example, at lag 2, we are calculating the correlation function of the two series `[0,1,2,3,4]` and `[2,3,4,5]`, where the positive correlation still holds. Where do I get this wrong?