In a class of 33 students, 22 swim and 22 play soccer every day. Everyone participates in at least one of the exercises every day. Of those who play soccer today, 15 swam and 15 played soccer yesterday, and the same situation applies to those who swim today. How many students swam only every day?
Let's calculate how many children played football over the two days. Today, 22 children played football, among them, 15 also played yesterday. Since 22 children played football yesterday as well, this means 7 of today's players were new. Therefore, $22+7=29$ children played at least once over the two days, and the remaining $33-29=4$ children swam both days.