24 Coins

You have 24 equally round coins. Can you put them on a table (each coin lying
flat on the table) such that each coin touches exactly three other coins?

The same question for 25 coins.

Can you generalize this? For what numbers of coins can this be done?
Source: Pertsel Vladimir, c/o internet newsgroup rec.puzzles.
Used as Ken's POTD 6/1/94.
Solution
Mail to Ken