Apple iOS 10 debuted the ability to send iMessages to other iPhone/iPad users with special effects, and there is a new one in iOS 11: Echo.
To use it, type your message, hold down the send key, and tap Screen up top.
Echo, which shows multiple echoes of your message (very effective with emoji), will be the first option. Choose one of the options.
I chose invisible ink. It is actually invisible until the recipient opens the message.