Oil is a bit of a loner in that it doesn’t really want to mix with other liquids. But vigorous whisking can overcome this anti-socialness. Think of mayonnaise: a blend of oil and egg; and a lemon dressing, in which oil is made to blend with citric acid. The science behind it is that very small droplets of each are dispersed in the other, but all you really need to know is that a good whisk equals a smooth blend.