How can I hide cell contents in Google Colaboratory?

1. You right-click on the area on the left of the cell (below the "Play" button) and choose "Add a form"
2. You can enter a title for your cell after the #@title keyword (first line in your cell code)
3. Right-click again in the same place and choose "Form > Hide code"

The black triangle, that makes it possible to fold and unfold sections, appears when you create a section (which is equivalent to creating a title).

You can create a section by creating a text cell that starts with # <your section title>.

This is how you create title in Markdown: # This is a title ## This is a smaller title ### This is even a smaller title...

You can fold and unfold sections by clicking on the triangle next to their title.

This image illustrates how to create a section:

This image is the result of the previous image (note the magic triangle on the left):

None of the other answers worked for me. You can collapse cells by adding this to the top of the cell "#@title" and then double clicking on the white space that appears to the right of it.

I realise this question was posted some time ago but as a newbie to Google Colab myself I thought it would be helpful to share what I have found useful when 'collapsing' sections of code and generally organising code within my notebooks.

To add a collapsible section: proceed title with '#', all code below will fall within this title and can be collapsed. I.e. '#Import libraries' will produce:

To add collapsible sub-sections (within the main header) simply add another '#', and so on. So, '##Check tf version' would fall within '#Import libraries'

I hope this is helpful!