@Alicia4Ever This isn’t one you can do within the Sandbox, I’m afraid. It’ll calve off a new little baby topic every time you try it. 
But this is still the best place to talk you through it if you come across another opportunity in the future. 
If you look in the top left corner of your reply window you’ll see a right-pointing arrow (and if you pressed the arrow at the bottom of this message to reply to me you should see my name up there too. Hello. 
).
This is who you’re directing your reply to. So either the individual you clicked reply to, or the Topic in general (if you click reply from the bottom of the topic).
There’s also a third option, which is to ‘Reply as linked Topic’. This splits off your imminent comment and turns it into a new Topic, with a link at the top showing where it originated.
Eg. The Trust Levels and Badges? Topic I split from Uploaded photos the other day. Have a look at the first post of that (and the link to where it split from) and you should see how it works, and when’s a good time to use it.