Why do we say 'science' with an /s/ but 'scan' with a /sk/? Both start with 'sc', yet one has a silent C. There is a clean rule that predicts which happens, and it depends entirely on the next letter.
The Rule
When 'sc' is followed by E, I, or Y, the C is silent and 'sc' is pronounced /s/. When 'sc' is followed by A, O, U, or a consonant, both letters are pronounced and 'sc' sounds like /sk/. This mirrors the soft/hard C rule you already know from single C.
Silent C: sc + E, I, Y = /s/
Full sound: sc + A, O, U = /sk/
Exceptions to Watch
'Muscle' is the famous rebel: the C is silent even though L follows, so it sounds like /ˈmʌsəl/. 'Sceptic' and 'scepter' keep the /s/ before E as expected. And be careful with words where 'sci' is fully spoken, like 'conscience' (/ˈkɑːnʃəns/), where 'sci' shifts to a /ʃ/ sound.
Practice Tip
Sort words by the letter after 'sc': science, scene, scent, scissors, descend (silent C) versus scan, scope, scarf, scream (full /sk/). For a related pattern, see our guide to the hard and soft C rule.