Why does my battery spark when I connect the cables

Why does my battery spark when I connect the cables When I try to start my truck everything comes on but it just clicks, I thought it was my starter so I dropped it and put it back on exactly the way I took it off, when I tried to reconnect the positve terminal I got a spark ...I did the same process with the negative terminal and got the sane result , now Im unable to connect my battery ....someone please give me legitimate advice thank you