SAN FRANCISCO (MarketWatch) -- Shares of Tesla Motors Inc. TSLA, +4.42% rose 4.3% at the open Thursday, hours after a cryptic tweet from CEO Elon Musk hinted at a new product or project, the "D", to be revealed on Oct. 9. Musk offered no further explanation on what the "D" might be. Tesla shares have gained 65% so far this year, but are down 13% from a record closing high on Sept. 4.