Rafael Nadal is not sure how his injured left knee will respond to his first hardcourt match of the year at the BNP Paribas Open.

Nadal is a two-time champion and three-time finalist at this event. He has not played a match on a hard court in almost a year and is only a month into his comeback from a left knee injury that sidelined him for seven months.

Willpower apparently can be bought. The chance to win or lose $20 a month enticed dieters in a yearlong study to drop an average of 9 pounds (4 kilograms)— four times more weight than others who were not offered dough to pass up the doughnuts.

Many employers, insurers and Internet programs dangle dollars to try to change bad habits like smoking or not exercising, but most studies have found this doesn't work very well or for very long.

The face of Mars is dotted with a maze of channels, pointing to possible ancient megaflood episodes.

Now scientists peering below the surface have uncovered the first evidence of underground channels apparently created by flooding — a finding that's expected to further illuminate the role of water in Mars' history.

Talk about a caffeine buzz: A new study says honeybees get a shot of caffeine from certain flowers, and it perks up their memory.

That spurs them to return to the same type of plant, boosting its prospects for pollination and the future of the plant species.

A new study looking at 11,000 years of climate temperatures shows the world in the middle of a dramatic U-turn, lurching from near-record cooling to a heat spike.

Research released Thursday in the journal Science uses fossils of tiny marine organisms to reconstruct global temperatures back to the end of the last ice age. It shows how the globe for several thousands of years was cooling until an unprecedented reversal in the 20th century.

On a dusty field in Israel's southern desert, the military is gearing up for the next battle against a familiar foe: Hizbullah fighters in Lebanon.

As the Syrian civil war intensifies, military planners are growing increasingly jittery that the fighting could spill over into Israel, potentially dragging the party that is allied with President Bashar Assad into the fray. After battling Hizbullah to a stalemate in 2006, the Israeli military says it has learned key lessons and is prepared to inflict heavy damage on the group if fighting begins again.

Leonardo DiCaprio says filming violent scenes like in "Django Unchained" doesn't deter him from wanting his movies to be great art.

The bloody revenge film about slavery before the U.S. Civil War has fueled some of the debate about whether Hollywood shows too much on-screen violence. DiCaprio portrays a ruthless plantation owner encountered by a freed slave and bounty hunter (Jamie Foxx and Christoph Waltz).

In Poland, speaking out in parliament is a privilege mostly reserved for lawmakers, but the opposition has skirted that regulation with the help of a hand-held tablet computer.

Nationalist opposition party leader and lawmaker Jaroslaw Kaczynski has introduced a non-confidence motion against the Cabinet of Prime Minister Donald Tusk, and a vote is expected Friday.

Justin Bieber is recovering after fainting backstage at a concert in London.

A spokeswoman for Bieber said late Thursday that the 19-year-old pop star was given oxygen and took a 20-minute reprieve after fainting backstage at London's O2 Arena.

Police in the Brazilian city of Sao Paulo are baffled by a macabre puzzle: someone has been leaving gift-wrapped human skulls around town.

Investigator Paul Henry Bozon Verduraz described the case to the Folha de Sao Paulo newspaper in a story published Thursday.