$\star$ Laser Beam $\star$

The answer is "Total Internal Reflection." Here's why it works:

By Snell's law, the path of a laser beam traveling through water (index $n_{\text{H}_2\text{O}} \approx 1.33$ ), making an angle $\theta$ normal to the boundary with the air (index $n_{\text{air}} \approx 1.0003$ ) will be totally reflected if $\sin\theta \geq \frac{n_{\text{air}}}{n_{\text{H}_2\text{O}}} \iff \theta \geq \sin^{-1}\left(\frac{n_{\text{air}}}{n_{\text{H}_2\text{O}}}\right) \approx 49^\circ$ This threshold is far enough below $90^\circ$ to trap a laser beam inside a small stream of water so that every angle of incidence the laser makes on the edges of the stream will be above that threshold, trapping the laser beam in the water.