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If you are part of multiple teams with your account, you can switch between them by clicking the team name in the upper left and choosing to “Sign into another team”. On mobile, you will see a four square icon in the upper right after tapping the team name instead of “Sign into another team”. Click any name under the “Channel” header in the left sidebar to change the chat area to that conversation.  You can also hit Ctrl+K to bring up a channel quick search. Tap the team name in the upper left to open the sidebar on mobile. Enter text into the text field and hit ↵ Enter to send. You can add emoji to your messages by with the smiley button on the right side of the text field (left side on mobile). You can surround parts (or all) of your text with certain symbols to change your formatting. A full list of formatting interactions can be found on the slack support site.  Using an asterisk (*) on each side of the message will display it in boldface. Using underscores (_) will italicize text. Using tildes (~) will strikethrough text. Use a triple backtick (```) on each side to format your text with a codebox. Click the three dot icon displayed on the right of a message when you hover the cursor over it. This will bring up a menu to edit, delete, mark unread, pin, or set a reminder for that message.  Tap and hold a message on mobile to bring up the full list of modifying options. Pinning a message will place it at the top of a channel and display it as you scroll. Use this for important announcements. You can select a period from 20 minutes to 1 week before the reminder takes place. Click the Star icon that appears next to the timestamp when hovering the cursor over the messages. This will add it to a list of saved messages that can be check later by clicking the Star icon in the upper right. On mobile, tap a message to select it, then tap the star that appears at the top of the screen. You can check your starred message by tapping the team name and selecting “Starred” from the dropdown menu in the upper left. Type “@” followed by the username of the person you want to mention. They will receive a slack notification that they have been mentioned in a chat.  For example: “@user: slack message”. You can also use mentions to address an entire channel or team (@channel, @team). You can check messages in which you’ve been mentioned by clicking the “@” icon in the upper right corner. On mobile, this is accessed by tapping the team name and selecting “Mentions” from the dropdown menu in the upper left. Click the smiley icon when hovering over a message to add an emoji reaction that will display directly below the message. These can be used for something like votes, or just for fun.  Reactions can also be added to messages from the modify message menu. On mobile this is the only way to add them. Reactions can only be emoji. Tap the “+” button in the chat field and select “Upload File” from the popup menu to browse your device for files.  You can also drag and drop files into the chat window on a computer. On mobile you can take and upload photos from the same chat field menu. Slack will store up to 5GB of files (including images) on its servers. This amount can be increased by upgrading to their paid services.
Switch between Teams. Switch between Channels. Send messages in chat. Use special formatting. Modify messages. Star messages to flag them as important. Mention a user in chat. Add reactions to messages. Upload files to chat.