What is the ruling on pretending to drink a cup of milk by concealing the cup and showing only a little of it to one's parents, in order to hide one's fasting?

The general principle is to conceal fasting and all other good deeds, based on the hadith: "Whoever among you can have a hidden store of good deeds, let him do so." This is because concealment is more conducive to sincerity and further removed from showing off, as Allah the Almighty says: "Invoke your Lord with humility and in secret." What you do, by pretending to drink milk to conceal your fasting, is permissible and falls under the category of ma'areedh (allusive speech), which provides a way out of lying, as the righteous predecessors did. It is important to note the obligation of obeying your parents if they prevent you from voluntary fasting out of compassion for you.