他们说双胞胎有感情，R也是吗？

Question

刚刚发现一个有趣的事情是对一个变量的操作，它会影响另一个双变量，当使用data.table时

library(data.table)
data("mtcars")
elder.brother <- data.table(mtcars)
paste("elder brother = ",length(elder.brother),sep="")

younger.brother <- elder.brother
paste("younger brother = ",length(younger.brother),sep="")

younger.brother[,c(1:10):= NULL]
paste("the younger brother = ",length(younger.brother), 
      " and the older brother = ",length(elder.brother), " too!",sep="")

当y被改变时，x也会改变，就像这个

> data("mtcars")
> elder.brother <- data.table(mtcars)
> paste("elder brother = ",length(elder.brother),sep="")
[1] "elder brother = 11"

> younger.brother <- elder.brother
> paste("younger brother = ",length(younger.brother),sep="")
[1] "younger brother = 11"

> younger.brother[,c(1:10):= NULL]
> paste("the younger brother = ",length(younger.brother), 
+       " and the older brother = ",length(elder.brother), " too!",sep="")

[1] "the younger brother = 1 and the older brother = 1 too!"

这是正常的吗？但是像我这样的菜鸟很奇怪。