Gmail asked me to solve mathematical problems

Few months back I composed mail using Gmail at midnight 11.00 PM. When I clicked Send button, 

Gmail asked me few simple mathematical problems. I am more interested to solve mathematical problems.I had just answered correctly. My mail had been sent. 

I tried second time with wrong answers, but Gmail asked me few other similar problems. Gmail allows the composed mail to be sent after giving correct answers.

When I send mail to the same friend next day morning, Gmail didn’t ask me the same questions. I had a 

question in my mind “Why Gmail is asking me mathematical problems only at night time?”

I never made effort to know answer for this question rather I assumed it’s for security reason.

Again Last week I used Gmail at late night. Gmail prompted the same kind of mathematical problems when I clicked send button. This time I want to go beyond my thinking and eager to know the answer for my long-lasting question. I got to know this is a Gmail Lab feature. This feature name is,

Mail Goggles:

In default this feature will be shown only on weekend “Friday” and “Saturday” from 10:00 PM to 4:00 AM.

We can change the day and time of this feature in ‘General’ tab in Settings.

Default 'Reply to all':

In default Gmail’s reply button replies only to the sender of the mail. If you want to send reply to all the recipients’ mail ids including those presented in CC. You can choose this feature.

Undo Send:

This is an interesting feature. I feel most of us need this feature. Within a few second after we pressed send button we think we made a mistake. But our mail would have gone. If we enable this feature Gmail will prompt us an option to do undo. This feature is disabled in default. We need to enable this feature in Gmail Labs.

Custom Keyboard shortcuts:

By enabling this feature, we can create our own shortcut for Gmail. After enabling this feature we can create shortcuts in “Mail Settings” --> “Keyboard Shortcuts” link.

To enjoy more Gmail Lab features Click the Settings Icon in the top right corner of the Gmail and Click Labs menu.