Now that smartphones and tablets are increasingly being used to browse the web, new captchas best suited to touch-capable devices should be devised, since entering text on soft keyboards is usually uncomfortable an error-prone. We contribute to solving this issue with μcaptcha, a novel captcha scheme to tell humans and computers apart by means of math handwriting input. Instead of entering text with a keyboard, the user retypes a mathematical expression on a touchscreen using e.g. the finger, a stylus, or an e-pen. Further, as a byproduct of solving μcaptcha challenges, a valuable labeled dataset of online handwritten math expressions is collected.