function Example(){
   const [a,setA] = React.useState(0);
   const m = React.useMemo(() => Math.random(),[]);
   const n = React.useCallback(() => Math.random(),[]);
   setTimeout(()=>{setA(a=>a+1);},1000);
   return (<React.Fragment>
                 <button>{m} (static)</button>
                 <button>{n()} (changing)</button>
              </React.Fragment>);}ReactDOM.render(<Example/>,document.querySelector("div"));