Key blocks destroy and recreate their contents when the value of an expression changes.

{#key number}
  <span style="display: inline-block" in:fly={{ y: -20 }}>
	{number}
  </span>
{/key}

This is useful if you want an element to play its transition whenever a value changes instead of only when the element enters or leaves the DOM.

Wrap the <span> element in a key block depending on number. This will make the animation play whenever you press the increment button.