surface.create_entity{name = "grenade",... doesn't work?

[yagaodirac]

When I throw a grenade, the surface.find_entities returns me a table with some entity named grenade. When I tried to create it from code, even if I specified the target, it still refused to work. The error info pointed out that a speed field is not specified, but I found no clue about it.
Any idea? How can I create a grenade entity?