If you want a fun way to automatically put your laptop to sleep (and lock the screen) with out closing the lid or touching the keyboard or trackpad, place it on top of a small magnet along the front, left corner.
However, you can also do this with another Mac laptop.
Example: Set down a running 2015 MacBook Air on top of a 2019 MacBook Pro 16" (closed and rotated 180-degrees) and watch the Air instantly sleep.
The reason for this is that the 2019 Pro added a magnet to the right end of the screen hinge. This is part of the new lid-angle sensor. When a post-2019 MacBook is rotated and underneath most pre-2019 Mac laptops with its older Hall-sensor (usually in the front, left corner), it causes the sleep function to kick in.
iFixit had an article about this. If you want some fun speculation as to Apple’s reason for the new sensor, read the last section of the article below and the comments. 
To replicate, set down a MacBook (running, unlocked) on top of a 2019 MacBook Pro 16" (lid closed) and immediately the top Mac’s screen and keyboard backlight will go dark. Attempting to wake the top laptop fails until it is lifted off of the Pro, and then the screen comes on. In other words, the 2019 Pro’s rear, right corner is near the top Mac’s front, left corner.
Confirmed with MacBook Air 2015, 2017 and MacBook Pro 2013, 2015. Theoretically this can work with most MacBook models through 2018 that use the older Hall lid sensor, if you have a newer MacBook made since the 2019 Pro 16" underneath.
Congratulations… In reading this you have now squandered a fraction of the time I wasted playing with this “hidden feature”.
If you really want to waste more time, read Hackaday’s piece on how some people are fighting for their lid sensor freedom: