What a fantastic episode, I loved it. Nina's sacrifice was tragic and heartbreaking to watch, but at the same time it is clear why she chose the course of action that she did. I applaud her character for the strength that she exhibited there and throughout the series. The Science Team's reaction was also heartbreaking, Walter's definitely being the most grief-stricken (where's that Emmy...) and Olivia's following from the close relationship that she shared with Nina. I absolutely loved Nina's speech to Windmark, about time somebody put that jerk in his place. The comparison of Observers to lizards was really well thought-out and made a great addition to the episode. I also really enjoyed the plot with the Observer child, and the scene where he cries for Nina after making an empathic connection with her was really powerful. The ending has given us some answers as well as raised some more questions. We know now who Donald is, but I feel like there is much more to that flashback sequence than what Walter said, and judging by the promo for 5x11, there are some interesting twists ahead. Excellent stuff, and if this is how the last few episodes are going to play out, I can't wait for the rest!