diff --git a/collects/scribblings/htdp-langs/intermediate.scrbl b/collects/scribblings/htdp-langs/intermediate.scrbl
index 3b4b5f3749..3498d6108a 100644
--- a/collects/scribblings/htdp-langs/intermediate.scrbl
+++ b/collects/scribblings/htdp-langs/intermediate.scrbl
@@ -164,7 +164,14 @@ This form is used to measure the time taken to evaluate
 @scheme[expr]. After evaluating @scheme[expr], Scheme prints out the
 time taken by the evaluation (including real time, time taken by the
 cpu, and the time spent collecting free memory) and returns the result
-of the expression.}
+of the expression.
+
+(The reported time is measured as the number of milliseconds of CPU time
+required to obtain this result, the number of “real” milliseconds required
+for the result, and the number of milliseconds of CPU time (included in the
+first result) spent on garbage collection. The reliability of the timing
+numbers depends on the platform.) 
+}
 
 @; ----------------------------------------------------------------------