diff --git a/process.c b/process.c
index cf5d824f90..05dee4521c 100644
--- a/process.c
+++ b/process.c
@@ -1401,6 +1401,9 @@ rb_fork(status, chfunc, charg)
  *  of its children or use <code>Process.detach</code> to register
  *  disinterest in their status; otherwise, the operating system
  *  may accumulate zombie processes.
+ *
+ *  The thread calling fork is the only thread in the created child process.
+ *  fork doesn't copy other threads.
  */
 
 static VALUE