Spaghetti is one of those things you learn how to make when you're young . . . because it's quick, easy, and you can eat leftovers for a week. But unless you know this simple trick, you've probably been doing it WRONG your whole life.
Instead of just straining the pasta when it's done, you should be SAVING some of that water, and adding it to the sauce.
Here's why . . .
You've probably noticed the pasta water gets really cloudy. But that doesn't mean it's "dirty" or bad. That cloudiness is just starch. And adding some of that starch to your sauce will THICKEN it because of something called emulsification.
If you ever end up with a thin puddly mess around the edge of your plate, it's because the oil and water in the sauce separated.
Starch is an emulsifying agent. Meaning it helps liquids and oils bind together. Adding some of that starchy water to your sauce will make it creamier. And it also helps the sauce stick to the pasta instead of sliding off.
Here's what to do: Before you strain the pasta, ladle out about a cup of water. Then slowly add some of it in while you're still cooking the sauce. You won't need it all. But if you add too much, just cook it a little longer so it evaporates off.