If you want to give yourself time to reply to a message on WhatsApp without the other person seeing the dreaded blue ticks, this trick will help. It lets you read a message without giving the game away.
Once you receive the message, don't open it, and dismiss any notifications on your home screen.
Open up Airplane mode, which blocks Wi-Fi and mobile data, and prevents any new messages from arriving
Open WhatsApp to read the message in full
Fully close the app and then deactivate Airplane mode. Despite having read the message, the blue ticks won't be displayed.
If read receipts are the bane of your WhatsApp existence, you can turn them off completely by going to Settings -> Account -> Privacy and toggle Read Receipts off.