The term 'swells (up)' can be defined by four crossword solutions: BALLOONS (8 letters), puffs (5 letters), bloats (6 letters), and bulks (5 letters). BALLOONS are typically made of rubber or plastic and filled with air or gas, causing them to swell up. Puffs are small amounts of air or gas that cause something to swell up. Bloats are when something swells up due to an excess of air or gas. Bulks are when something swells up due to an increase in size or mass.