Sentence:  ‘My very creative 2-year-old son used my good friend′s white polyester Roman blinds as a canvas,' says Jacqui. They′re now covered in blue <WSD>biro</WSD>. And although my friend isn′t too bothered by the <WSD>biro</WSD>, I am! Can you help?′ 
Meaning of the word marked with <WSD> tags:  Ink from a ballpoint pen.