DFT tutoring: students confuse cycle count with bar count

When teaching DFT plots, a student who correctly says '6 Hz means 6 cycles in 1 s' may still conclude there are 6 bars spaced 1/6 apart, because the period in time gets carried onto the k axis. Drawing the time graph directly above the |X[k]| graph fixed it: the cycle count is the bar's position, and one real tone always gives one pair at k and N-k. A related slip is treating the sampling rate as the test wave frequency. Separate them explicitly: spacing = 1/(record length), limit = f_s/2.