The chemistry wordplay here is absolutely brilliant! "NaH" is the chemical formula for sodium hydride, but it's also pronounced like "nah" - the casual way of saying "no." So when asked if spilling sodium hydride hurts, the chemist responds with "NaH" - simultaneously saying "no" and naming the compound!
Fun fact: Real sodium hydride would actually be super dangerous to spill on your hand - it violently reacts with moisture (including sweat) to produce hydrogen gas and sodium hydroxide (which is caustic). Chemistry jokes are way safer than chemistry accidents!