A colander sorts by size—and by gravity
The colander’s job is to let water escape without letting dinner follow. Its holes are smaller than the pasta pieces, while their combined open area gives a large volume of water many routes out. As the pot is tipped, gravity pulls the water through; the pasta is held back by the rim and the surrounding mesh or perforated bowl.
Drainage is fastest while there is plenty of water pressing toward the openings. Near the end, the remaining water is spread among the noodles and clings to their surfaces, so lifting or gently shaking the colander helps it run off. The holes also have to suit the pasta: openings that work for rigatoni could let orzo slip through.
The useful tradeoff happens before draining
A colander can separate pasta from cooking water, but it cannot separate the pasta from substances dissolved in that water. The cloudy liquid contains starch released during cooking, which can help a sauce cling and come together. Setting aside some water before tipping the pot preserves that option; once it has passed through the holes, it is gone.
That makes the colander a simple size-based separator, not a selective filter: it keeps the solid pieces and sends the liquid—and anything dissolved in it—down the drain.
