You are given a mixture of oil and water. Which separation technique would you use to separate them? Explain the principle behind this technique.

To separate a mixture of oil and water, I would use a separating funnel. The principle behind this technique is that oil and water are immiscible liquids, meaning they do not mix and form separate layers due to differences in their densities. 

Oil, being less dense, forms the top layer, while water, being denser, forms the bottom layer. The separating funnel allows the heavier (bottom) layer to be drained off carefully, leaving the lighter (top) layer behind.