It's an optical illusion created by the bending of light though the water.
The mass of a copper coin can vary depending on the size and denomination of the coin. On average, a copper coin like a US penny weighs about 2.5 grams.
From 1965 to date, U.S. quarters weigh 5.67 grams.
All US Nickels weigh the same, 5 grams.
The US nickel (5 cents) weighs 5 grams.
The sum of these two masses is 13.608g.
No, no US coin is light enough to float when dropped in water.
A two cent US coin is larger in size than a ten cent US coin.
Sacagawea is on the US $1 coin.
They never have. If your coin has a hole and is a US coin then someone drilled it in. It was not done at a US mint.
The first US dollar coin was made in 1794.
You are probably thinking of an "error coin". This coin is any coin which does not meet the standards of the US Mint or has a flaw in its design or manufacture.
In fact, coin designs don't have to be raised. Most are because it's easier to strike them in what's called "relief" with the image raised against a flat background. In addition it's easier for the eye to interpret a relief design, and to some extent it's simply traditional. However some coins are struck with what's called "incuse" designs: the design is sunk into the coin below the surface so that the background is the highest part of the design. Two US coins, the famous $2.50 and $5.00 Indian head gold pieces, were incuse. There's a picture at the Related Link
Harriet Tubman has never been on a US Mint Coin.
...Of what US coin? There have been several US coins intended for circulation ranging from half a cent to $20.
A 1941 Republique Francaise 50 centimes is worth between 1 and 8 US dollars. The value depends on the condition of the coin.
Take it to a coin shop. They will tell you if it is real or not.
The penny coin which was worth a cent