Unknown iOS Messages feature proclaimed the greatest Apple feature of all time by one Redditor،
To use the Apple feature, you must use iMessage. First, send an emoji to a recipient, choose another emoji and drag it over the emoji you already sent. You can add multiple emoji and even use a second finger to drag outward to make a second emoji larger than the first. You can also rotate any of the emojis by rotating your finger on the screen. Based on comments made on RedditIt seems that most iPhone users are unaware that this feature is available.
Some examples showing how iOS users can combine emoji to create stickers on iMessage
More examples from the recipient's perspective
Keep in mind that this feature will only work if you send the first emoji to your recipient. Once you remove your finger from the second emoji (or however many additional emojis you use), the combined emojis will be delivered to the recipient as a sticker. If you're ambitious, you can tell a whole story to a friend or family member with the creative use of emojis.