Stopping $K^{+}$ in the CERN heavy liquid bubble chamber. The photo shows the body of the chamber as seen by a camera. The enlargement (centre) and its graphic explanation show an example of a $\tau$ decay of a $K^{+}$, ie. $K^{+} \rightarrow \pi^{+} \pi^{+} \pi^{-}$. The 3 pions are seen to stop. The $\pi^{-}$ is absorbed in a nucleus whereas the $\pi^{+}$ decay first into $\mu{+}$ and then in $e^{+} \nu \overline{\nu}$. As can be seen from the photograph, the muons are visible. The visibility of the decay of $\pi \rightarrow \mu$ was an essnetial feature of the X2 and K5 experiments, in order to eliminate background in the study of the $K^{+} \rightarrow \mu^{+} + \pi^{0} + \nu$ and $K^{+} \rightarrow e^{+} + \pi^{+} + \pi^{-} + \nu$ decay modes

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