How can you leave a room with two legs and return with six legs?

This puzzle is for Harry Potter fans. Do you know why Tom Riddle is called Lord Voldemort?

I have cities, but no houses. I have mountains, but no trees. I have water, but no fish. What am I?

I am a prime number.
The double of myself is equal to the square of me.

which number am I?

Which is the rope you never skip with?

If I put in one bird per cage, I have one bird too many. If I put in two bird per cage, I have one cage too many. How many cages and birds do I have?

A shepherd had 18 sheep, out of which, all but ten died.

Can you tell us how many sheep was he left with?

Prove that twice of ten is the same as twice of eleven.

How many letters are in the alphabet?

Let's play a word game. Following are some meaningless words that can be completed by adding random letters on either side of the word.

For example, the word "rdo" can be formed into "pardon".

Using the same technique, can you complete the given words?
ecalc
airb
rinci
tibac
uidel
gorit
igna
eyho