Ch03 - Basic service is not working as expected.

[realalien]

After creating a service with following code,

apiVersion: v1
kind: Service
metadata:
  name: sleep-2
spec:
  selector:
    app: sleep-2
  ports:
    - port: 80

the command kubectl exec deploy/sleep-1 -- ping -c 1 sleep-2 will throw error like ping: bad address 'sleep-2' command terminated with exit code 1 ,
while the kubectl exec deploy/sleep-1 -- ping -c 2 $(kubectl get pod -l app=sleep-2 --output jsonpath='{.items[0].status.podIP}') works just fine.

The manual of Kubernetes Service is hard to digest for noobie like me, what's the cause?